…ame#8911)

As discussed in MonoGame#8899, here's
a simple .editorconfig to start with.

1) XML Summary mistakes, except for ''XML summaries are missing
entirely'' are now marked as errors, not warnings; this is because it
can brick exports to the documentation website, and besides, a good API
contract requires proper formatting.
2) Added a few other common issues as warnings.

This has been tested against all solutions to check for errors that now
spring up; a few malformed XML summaries were detected and dealt with:
1) Color.ToHS didn't have documentation for all its parameters
2) TextInputEventArgs' XML summary fails in Android builds because the
element it's pointing at does not exist in that configuration (due to
macro #if and #endif statements)
3) ImmutableVertexInputLayout refers to something called
InputLayoutCache which no longer exists. Well,
ImmutableVertexInputLayout itself is internal and no longer in use, so
it might be an idea to remove it entirely. But that's not in the scope
of this task nor do I know if it may be used elsewhere, so I left it be.
4. A number of classes in the test suite were without a namespace.
However, as those are only there for performing a number of unittests,
they're #pragma disabled in this case.
5. A number of XML summaries in the content pipeline had incorrect
references to parameters or to an enum value that has since been
renamed.

**As a result of this PR, you might notice _a ton more warnings popping
up. This is fine._**

Examples:
<img width="1107" height="417" alt="image"
src="https://github.com/user-attachments/assets/d79103d4-592f-4bcd-96ab-560093d6b8fa"
/>

In this case, the string is serialized based on the culture of the local
machine, which causes comma's as the delimiter on many European desktops
and dots in invariant cultures or US desktops. In this case it's
relatively harmless, but this is the pinnacle of "but it works on my
machine!" bug.

<img width="694" height="320" alt="image"
src="https://github.com/user-attachments/assets/95287a9c-65b7-4a15-a92d-940ed864950a"
/>

In this case it might be less harmless, based on what the user is doing
with the output. In most cases, a simple `CultureInfo.InvariantCulture`
ensures the same behavior on all machines.