Add grammar doc generator and doc-comments to grammar.y

Add a script (meta/src/doc-gen/grammar_to_markdown.py) that parses
grammar.y and generates a clean Markdown reference of the S-expression
syntax, stripping construct/deconstruct actions and protobuf details.
Non-terminal references in production bodies link to their definitions.

Add doc-comments (# ...) to all grammar rules in grammar.y. Comments
immediately above a rule name are extracted by the doc generator and
rendered as descriptions in the output.

Generate docs/grammar.md from the annotated grammar.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: staworko