Docs: Add simplified block grammar spec to handbook

[mcsf]

Description

Add docs/grammar.md, a document auto-generated since #6116 containing a simplified specification of the Gutenberg block grammar, to the Gutenberg Handbook.

How has this been tested?

Changes to the handbook aren't very testable beforehand. Once these changes are merged and applies, the expectation is that:

• The document renders properly at https://wordpress.org/gutenberg/handbook/language/grammar
• The links added in this PR between the simplified spec and the parsing packages work both in the Handbook and in GitHub.

Types of changes

Documentation.

[pento]

📖

[pento]

I had to make a couple of tweaks to the docs importer: https://meta.trac.wordpress.org/changeset/7668

[mcsf]

I had to make a couple of tweaks to the docs importer

Thanks!

[aduth]

When I run npm run docs:build, it removes the "grammar" entry from manifest.json.

[mcsf]

When I run npm run docs:build, it removes the "grammar" entry from manifest.json.

Oh, because I'm an idiot who hadn't realized root-manifest.json is the cool kids' place now. Fix in #10014.