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/