Add testing and debugging guide page to docs

[andfoy]

This adds a documentation page that highlights some of the steps for how to go about testing free-threading Python for a library.

[rgommers]

Thanks @andfoy! This looks like it covers a good amount of ground for testing/debugging.

The one thing that comes to mind as nice to add is a note on hitting static_assert inside CPython, and what that could mean. @ngoldbaum it looks like you're going to review this PR in more detail? If so, maybe you could add this point?

[ngoldbaum]

Left some text suggestions and comments.

@rgommers agreed we need a section covering common error messages for google to index. If you don't mind I'll go ahead and add a FAQ section in a followup.

[lysnikolaou]

Thanks @andfoy! Ralf and Nathan covered most of it, but I left a couple of comments as well.

[rgommers]

@rgommers agreed we need a section covering common error messages for google to index. If you don't mind I'll go ahead and add a FAQ section in a followup.

SGTM!

[ngoldbaum]

the updated text looks good!

[rgommers]

Very nice, thanks @andfoy! And thanks @ngoldbaum and @lysnikolaou for the reviews.