gh-101100: Fix sphinx warnings in Doc/library/__future__.rst

[sobolevn]

Before:

/Users/sobolev/Desktop/cpython/Doc/library/__future__.rst:54: WARNING: py:class reference target not found: _Feature /Users/sobolev/Desktop/cpython/Doc/library/__future__.rst:54: WARNING: py:meth reference target not found: getOptionalRelease /Users/sobolev/Desktop/cpython/Doc/library/__future__.rst:54: WARNING: py:meth reference target not found: getMandatoryRelease /Users/sobolev/Desktop/cpython/Doc/library/__future__.rst:57: WARNING: py:attr reference target not found: compiler_flag /Users/sobolev/Desktop/cpython/Doc/library/__future__.rst:57: WARNING: py:class reference target not found: _Feature 

Changes:

1. I documented _Feature class and its members and added sphinx markup, because they were already documented but without the proper markup

This is now much more readable, has more metadata, and links to every part of the explanation.

2. I've also changed

MandatoryRelease may also be None, meaning that a planned feature got
dropped.

to be

MandatoryRelease may also be None, meaning that a planned feature got
dropped or that it is not yet decided.

Because of the note below:

No final decision has been made yet. See also :pep:563 and :pep:649.

• Issue: Fix all Sphinx reference warnings in the documentation #101100

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📚 Documentation preview 📚: https://cpython-previews--109814.org.readthedocs.build/

[miss-islington-app]

Thanks @sobolevn for the PR, and @hugovk for merging it 🌮🎉.. I'm working now to backport this PR to: 3.11.
🐍🍒⛏🤖

[miss-islington-app]

Thanks @sobolevn for the PR, and @hugovk for merging it 🌮🎉.. I'm working now to backport this PR to: 3.12.
🐍🍒⛏🤖

[miss-islington-app]

Sorry, @sobolevn and @hugovk, I could not cleanly backport this to 3.12 due to a conflict.
Please backport using cherry_picker on command line.

cherry_picker f2eaa92b0cc5a37a9e6010c7c6f5ad1a230ea49b 3.12 

[bedevere-app]

GH-112701 is a backport of this pull request to the 3.11 branch.

[bedevere-app]

GH-112702 is a backport of this pull request to the 3.12 branch.