- this removes one external dependency, mdbook-toc
- this steals code from rustc book

Update documentation for current tests requirements

make toc generation fully automatic

there is still no official policy regarding new rustc crate deps

Rollup of 18 pull requests

Successful merges:

 - #144467 (rustdoc template font links only emit `crossorigin` when needed)
 - #144548 (Rehome 21 `tests/ui/issues/` tests to other subdirectories under `tests/ui/`)
 - #144596 (libtest: print the type of test being run)
 - #144741 (fix: Error on illegal `[const]`s inside blocks within legal positions)
 - #144776 (`Printer` cleanups)
 - #144779 (Implement debugging output of the bootstrap Step graph into a DOT file)
 - #144813 (Add a tidy check to prevent adding UI tests directly under `tests/ui/`)
 - #144817 (Properly reject tail calls to `&FnPtr` or `&FnDef`)
 - #144852 (Rename `rust_panic_without_hook` to `resume_unwind` )
 - #144866 (Remove `SHOULD_EMIT_LINTS` in favor of `should_emit`)
 - #144867 (Use `as_array` in PartialEq for arrays)
 - #144872 (Document Poisoning in `LazyCell` and `LazyLock`)
 - #144877 (coverage: Various small cleanups)
 - #144887 (`rust-analyzer` subtree update)
 - #144890 (Add `InterpCx::project_fields`)
 - #144894 (Delete `tests/ui/threads-sendsync/tcp-stress.rs`)
 - #144905 (rustc-dev-guide subtree update)
 - #144920 (Dont print arg span in MIR dump for tail call)

r? `@ghost`
`@rustbot` modify labels: rollup

Rollup of 17 pull requests

Successful merges:

 - #144467 (rustdoc template font links only emit `crossorigin` when needed)
 - #144548 (Rehome 21 `tests/ui/issues/` tests to other subdirectories under `tests/ui/`)
 - #144741 (fix: Error on illegal `[const]`s inside blocks within legal positions)
 - #144776 (`Printer` cleanups)
 - #144779 (Implement debugging output of the bootstrap Step graph into a DOT file)
 - #144813 (Add a tidy check to prevent adding UI tests directly under `tests/ui/`)
 - #144817 (Properly reject tail calls to `&FnPtr` or `&FnDef`)
 - #144852 (Rename `rust_panic_without_hook` to `resume_unwind` )
 - #144866 (Remove `SHOULD_EMIT_LINTS` in favor of `should_emit`)
 - #144867 (Use `as_array` in PartialEq for arrays)
 - #144872 (Document Poisoning in `LazyCell` and `LazyLock`)
 - #144877 (coverage: Various small cleanups)
 - #144887 (`rust-analyzer` subtree update)
 - #144890 (Add `InterpCx::project_fields`)
 - #144894 (Delete `tests/ui/threads-sendsync/tcp-stress.rs`)
 - #144905 (rustc-dev-guide subtree update)
 - #144920 (Dont print arg span in MIR dump for tail call)

r? `@ghost`
`@rustbot` modify labels: rollup