WIP: Docs: use a Sphinx extension to eliminate excessive links

[nedbat]

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📚 Documentation preview 📚: https://cpython-previews--145130.org.readthedocs.build/

[nedbat]

I had a good idea: instead of writing a linter to change .rst files (pinging everyone, expecting reviews, hardcoding what's excessive, etc), now we have a Sphinx extension that does it all on the fly during the build.

[nedbat]

Of course: if we like this, I'll publish linklint for real.

[nedbat]

Some methods aren't unlinking properly, like ZipFile.close, but I think that's because the .rst is wrong: the methods aren't indented under the class. Is that a mistake in the .rst? Sphinx understands that "close" is "ZipFile.close", or is that because the .rst has .. method: ZipFile.close() ?

[nedbat]

I see many classes document their methods that way, so there's more work to do.