RST markup: recommend anonymous hyperlinks

[hugovk]

To avoid potential clashes in target names, let's recommend "anonymous hyperlinks", which use a double trailing underscore instead of a single underscore.

Re: python/cpython#142057 (comment)

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Not in this PR, updating all the other hyperlinks in the devguide would look like this: hugovk@072c3da

Worth including or not?

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📚 Documentation preview 📚: https://cpython-devguide--1709.org.readthedocs.build/

[picnixz]

If you use anonymous links in markup.rst, let's include the others as well I guess. I think it would make more sense to have one of the following:

• 1 PR for recommendation, 1 PR for refactoring.
• 1 PR for both of them

But putting some refactoring in the recommendation feels weird..

[hugovk]

@picnixz Which option do you want?

[picnixz]

Really, it's up to you. I think it's fine to include your other commit :') less noise, less approval needs

[picnixz]

(I've reviewed all files, well not in details, but AFAICT, everything is fine, thank you!)