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

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

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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>

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