gh-127833: Docs: Add a grammar-snippet directive & replace productionlist

[encukou]

As a first step to what's proposed in the issue, this overrides the ReST productionlist directive to:

• use : instead of the ::= symbol
• add syntax highlighting for strings (the implementation is preliminary; I intend to later add a proper class to the theme instead of borrowing a Pygments class.)

All links and link targets should be preserved. (Unfortunately, this reaches into some Sphinx internals; I don't see a better way to do exactly what Sphinx does.)

This also adds a new directive, grammar-snippet, which formats the snippet almost exactly like what's in the source, modulo syntax highlighting and keeping the backtick character to mark links to other rules.
This will allow formatting the snippets as in the grammar file (most importantly: to remove the the indentation that Sphinx adds to align the text).

I'd appreciate if a Sphinx expert could take a look to see if there's a better way to do something :) cc @AA-Turner

• Issue: Align the grammar documentation with Python's actual grammar #127833

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📚 Documentation preview 📚: https://cpython-previews--127835.org.readthedocs.build/

[AA-Turner]

A few initial notes. The targets problem is annoying, Sphinx doesn't make it easy...

[AA-Turner]

I'm happy with this as it stands, thank you @encukou for the discussion. As discussed, I've pushed a commit to add type annotations. I've included some other minor changes and left a self-review for a brief explanation -- feel free to revert any you disagree with.

A

[miss-islington-app]

Thanks @encukou for the PR 🌮🎉.. I'm working now to backport this PR to: 3.12, 3.13.
🐍🍒⛏🤖

[bedevere-app]

GH-129689 is a backport of this pull request to the 3.13 branch.

[bedevere-app]

GH-129690 is a backport of this pull request to the 3.12 branch.