Docs: Add central references to free-threading-related options

[swtaarrs]

The docs for Py_mod_gil said "See PYTHON_GIL and -X gil <-X> for more detail," but neither of those gave any detail about loading extensions in a free-threaded build.

• Replace that sentence with a reference to the summary in the 3.13 what's new doc.
• Add a few more links to that summary in other related areas.
• Fix a syntax error in the versionadded:: line for Py_mod_gil that was causing it to not get rendered.

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