gh-127833: Fix grammar snippet formatting for help() output

[encukou]

Change pydoc_data to show the "input" of grammar snippet
directives, which is formatted the way we want.

See also this Sphinx issue: sphinx-doc/sphinx#13302
And: sphinx-doc/sphinx#4454

A known issue is cross-grammar links, like those in
the string format specification mini-language, which render
as reST source:

element_index: `~python-grammar:digit`+ | `index_string`

I don't see a good way to fix this before Sphinx is refactored.

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

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📚 Documentation preview 📚: https://cpython-previews--129692.org.readthedocs.build/

[hugovk]

(re: https://github.com/python/cpython/actions/runs/13162643246/job/36735040454?pr=129692, I've updated the branch from main to pull in the new change detection (#129627))

[encukou]

Thank you!
Does the change look good now?

[encukou]

This PR was made obsolete by new Sphinx and #130376. Thank you Adam!