docs: migrate docs UI to Astro Starlight

[joshua-temple]

Problem

The docs site runs on MkDocs Material, which diverges from the rest of the suite. Several ASCII diagrams in the content are misaligned, and the README badge block and brand art are inconsistent with the suite convention.

Fix

• Replace the MkDocs Material site with an Astro Starlight stack (astro, @astrojs/starlight, astro-mermaid), mirroring the suite docs setup.
• Apply the cascade brand: cyan-teal (#36D0C4) accent tokens, dark-default theme with a light toggle, PNG favicon and upper-left logo, hero and avatar art.
• Port every docs page into the Starlight content collection with frontmatter, preserving the previous nav order in the sidebar. Convert internal links to Starlight routes.
• Realign the system-overview and primary/satellite ASCII diagrams, and render the manifest state machine as a Mermaid statechart.
• Retire mkdocs.yml, the docs requirements file, and the old Pages workflow; replace them with an Astro build + Pages deploy workflow.
• Reorder the README badges to Go Reference, Go Report Card, Go Version, Docs, and add the avatar at the top and the hero in the overview.

Verification

• cd docs && npm install && npm run build builds all 9 pages clean (mermaid block transformed, hero optimized, search index built).
• actionlint .github/workflows/docs-deploy.yaml passes.
• No doc content lost: all eight pages ported with line counts intact.