Add initial free-threading page for the guide

[ngoldbaum]

Moves the existing content on free-threading in the migration guide into its own page. Also adds some new content about things we know are going to be issues for some users.

Comments and suggestions are very welcome. I'd really appreciate ideas for illustrative code examples to add, if anyone has any.

[ngoldbaum]

Thanks for all the comments! I hope the new text addresses the concerns and is clearer.

I'd like to add sections on the critical section and PyMutex wrappers, assuming those get merged, and also maybe another section in the docs about multithreaded programming using PyO3? I definitely found it nontrivial to learn and things like std::thread::scope should be pointed at explicitly in the PyO3 docs.