Fixes part of RaspberryPiFoundation/blockly#3370
Fixes part of RaspberryPiFoundation/blockly#9283

This PR introduces the plugin corresponding changes needed by RaspberryPiFoundation/blockly#9280.

In particular, this:
- Introduces CI workflow updates so that "tip-of-tree" is redefined as the corresponding experimental branch in core for screen reader support (to ensure CI passes here for changes that happen there).
- Removes some access error overrides since the corresponding field was made public in the core PR (which is the root of the CI failures here).
- Introduces some general instructions in the navigation plugin playground for folks testing Blockly with a screen reader (and points to #673 for discussion).

**Important**: Note that this PR's changes will NOT be merged into the `main` branch as-is. They will be reconsidered entirely (and likely largely reimplemented) separately from this experimental branch and submitted as a separate, future review for `main`. See RaspberryPiFoundation/blockly#9280 and Fixes part of RaspberryPiFoundation/blockly#9283 for more specifics on why the separate branch thing is being set up this way.

* feat: Deploy GH pages w/ screen reader experiment.

* fix: Use correct branch for plugin.

* feat: Introduce initial screen reader support  (experimental) (RaspberryPiFoundation#684)

Fixes part of RaspberryPiFoundation/blockly#3370
Fixes part of RaspberryPiFoundation/blockly#9283

This PR introduces the plugin corresponding changes needed by RaspberryPiFoundation/blockly#9280.

In particular, this:
- Introduces CI workflow updates so that "tip-of-tree" is redefined as the corresponding experimental branch in core for screen reader support (to ensure CI passes here for changes that happen there).
- Removes some access error overrides since the corresponding field was made public in the core PR (which is the root of the CI failures here).
- Introduces some general instructions in the navigation plugin playground for folks testing Blockly with a screen reader (and points to RaspberryPiFoundation#673 for discussion).

**Important**: Note that this PR's changes will NOT be merged into the `main` branch as-is. They will be reconsidered entirely (and likely largely reimplemented) separately from this experimental branch and submitted as a separate, future review for `main`. See RaspberryPiFoundation/blockly#9280 and Fixes part of RaspberryPiFoundation/blockly#9283 for more specifics on why the separate branch thing is being set up this way.

* feat: Deploy GH pages w/ screen reader experiment.

* fix: Use correct branch for plugin.

* feat: Introduce initial screen reader support  (experimental) (RaspberryPiFoundation#684)

Fixes part of RaspberryPiFoundation/blockly#3370
Fixes part of RaspberryPiFoundation/blockly#9283

This PR introduces the plugin corresponding changes needed by RaspberryPiFoundation/blockly#9280.

In particular, this:
- Introduces CI workflow updates so that "tip-of-tree" is redefined as the corresponding experimental branch in core for screen reader support (to ensure CI passes here for changes that happen there).
- Removes some access error overrides since the corresponding field was made public in the core PR (which is the root of the CI failures here).
- Introduces some general instructions in the navigation plugin playground for folks testing Blockly with a screen reader (and points to RaspberryPiFoundation#673 for discussion).

**Important**: Note that this PR's changes will NOT be merged into the `main` branch as-is. They will be reconsidered entirely (and likely largely reimplemented) separately from this experimental branch and submitted as a separate, future review for `main`. See RaspberryPiFoundation/blockly#9280 and Fixes part of RaspberryPiFoundation/blockly#9283 for more specifics on why the separate branch thing is being set up this way.

* feat: Deploy GH pages w/ screen reader experiment.

* fix: Use correct branch for plugin.

* feat: Introduce initial screen reader support  (experimental) (RaspberryPiFoundation#684)

Fixes part of RaspberryPiFoundation/blockly#3370
Fixes part of RaspberryPiFoundation/blockly#9283

This PR introduces the plugin corresponding changes needed by RaspberryPiFoundation/blockly#9280.

In particular, this:
- Introduces CI workflow updates so that "tip-of-tree" is redefined as the corresponding experimental branch in core for screen reader support (to ensure CI passes here for changes that happen there).
- Removes some access error overrides since the corresponding field was made public in the core PR (which is the root of the CI failures here).
- Introduces some general instructions in the navigation plugin playground for folks testing Blockly with a screen reader (and points to RaspberryPiFoundation#673 for discussion).

**Important**: Note that this PR's changes will NOT be merged into the `main` branch as-is. They will be reconsidered entirely (and likely largely reimplemented) separately from this experimental branch and submitted as a separate, future review for `main`. See RaspberryPiFoundation/blockly#9280 and Fixes part of RaspberryPiFoundation/blockly#9283 for more specifics on why the separate branch thing is being set up this way.

* Revert "feat: Introduce initial screen reader support  (experimental) (RaspberryPiFoundation#684)"

This reverts commit 458e4f8.

* feat: Deploy GH pages w/ screen reader experiment.

* fix: Use correct branch for plugin.

* feat: Introduce initial screen reader support  (experimental) (RaspberryPiFoundation#684)

Fixes part of RaspberryPiFoundation/blockly#3370
Fixes part of RaspberryPiFoundation/blockly#9283

This PR introduces the plugin corresponding changes needed by RaspberryPiFoundation/blockly#9280.

In particular, this:
- Introduces CI workflow updates so that "tip-of-tree" is redefined as the corresponding experimental branch in core for screen reader support (to ensure CI passes here for changes that happen there).
- Removes some access error overrides since the corresponding field was made public in the core PR (which is the root of the CI failures here).
- Introduces some general instructions in the navigation plugin playground for folks testing Blockly with a screen reader (and points to RaspberryPiFoundation#673 for discussion).

**Important**: Note that this PR's changes will NOT be merged into the `main` branch as-is. They will be reconsidered entirely (and likely largely reimplemented) separately from this experimental branch and submitted as a separate, future review for `main`. See RaspberryPiFoundation/blockly#9280 and Fixes part of RaspberryPiFoundation/blockly#9283 for more specifics on why the separate branch thing is being set up this way.

* Revert "feat: Introduce initial screen reader support  (experimental) (RaspberryPiFoundation#684)"

This reverts commit 458e4f8.

* chore: Remove unnecessary change dirs.

…(experimental) (#9284)

## The basics

- [x] I [validated my changes](https://developers.google.com/blockly/guides/contribute/core#making_and_verifying_a_change)

## The details
### Resolves

Fixes #9283

### Proposed Changes

Updates the keyboard navigation plugin workflow to point to the screen reader experimentation branch.

### Reason for Changes

This ensures that both the plugin (via RaspberryPiFoundation/blockly-keyboard-experimentation#684) and core Blockly correctly link their experimental screen reader branches against each other to ensure CI passes correctly for both.

### Test Coverage

No new tests are needed here.

### Documentation

N/A

### Additional Information

Similar to #9280, this PR will never be merged directly into `develop` so the workflow changes will remain isolated to the experimental branch.