[3.13] gh-127833: Docs: Add a grammar-snippet directive & replace productionlist (GH-127835)

[miss-islington]

As a first step toward aligning the grammar documentation with Python's actual
grammar, this overrides the ReST productionlist directive to:

• use : instead of the ::= symbol
• add syntax highlighting for strings (using a Pygments highlighting 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
(file:///home/encukou/dev/cpython/Doc/build/html/reference/grammar.html).

The new directive is applied to two simple rules in toplevel_components.rst

---

(cherry picked from commit 58a4357)

Co-authored-by: Petr Viktorin encukou@gmail.com
Co-authored-by: Blaise Pabon blaise@gmail.com
Co-authored-by: William Ferreira wqferr@gmail.com
Co-authored-by: bswck bartoszpiotrslawecki@gmail.com
Co-authored-by: Adam Turner 9087854+aa-turner@users.noreply.github.com

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

---

📚 Documentation preview 📚: https://cpython-previews--129689.org.readthedocs.build/

[encukou]

Let's not merge the backport just yet. I'd like to see this change go through the release process first.

[encukou]

It should be merged together with a follow-up PR, #129692. If at all.

[AA-Turner]

#130376 fixes the help() output. I think we should backport, especially as this PR is a pre-requisite for all of the work done on improving how we describe the grammar in the documentation. What do you think @encukou?

A

[encukou]

I'm considering to backporting this for the syntax highlighting, but the upcoming grammar docs improvements are another matter. Those are tied to the current grammar, verifying them is a fiddly manual process, and they're interlinked in complex ways -- a token in some grammar rule might need to move to another rule in another file, which Git won't catch as a conflict, and the nonexistent tests won't either (of course I'd like to add tests, but that does require more automation, and in the current approach, rewriting the prose needs to come first.)