feat(docs-site): scaffold Astro Starlight site

Scaffold docs/site/ via `npm create astro@latest ... --template
starlight`. Extend the docs collection schema in
src/content.config.ts with the four custom frontmatter fields (man,
site, manTitle, helpKeywords) needed by the generator in a later
task, using z.strictObject so unrecognized keys fail the build
instead of being silently stripped by Zod's default behavior.

Ignore generated site output (node_modules, dist, .astro, generated
content, and sidebar.json) in .gitignore.

docs/site/public/favicon.svg

@@ -0,0 +1 @@
+<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 128 128"><path fill-rule="evenodd" d="M81 36 64 0 47 36l-1 2-9-10a6 6 0 0 0-9 9l10 10h-2L0 64l36 17h2L28 91a6 6 0 1 0 9 9l9-10 1 2 17 36 17-36v-2l9 10a6 6 0 1 0 9-9l-9-9 2-1 36-17-36-17-2-1 9-9a6 6 0 1 0-9-9l-9 10v-2Zm-17 2-2 5c-4 8-11 15-19 19l-5 2 5 2c8 4 15 11 19 19l2 5 2-5c4-8 11-15 19-19l5-2-5-2c-8-4-15-11-19-19l-2-5Z" clip-rule="evenodd"/><path d="M118 19a6 6 0 0 0-9-9l-3 3a6 6 0 1 0 9 9l3-3Zm-96 4c-2 2-6 2-9 0l-3-3a6 6 0 1 1 9-9l3 3c3 2 3 6 0 9Zm0 82c-2-2-6-2-9 0l-3 3a6 6 0 1 0 9 9l3-3c3-2 3-6 0-9Zm96 4a6 6 0 0 1-9 9l-3-3a6 6 0 1 1 9-9l3 3Z"/><style>path{fill:#000}@media (prefers-color-scheme:dark){path{fill:#fff}}</style></svg>