feat: build the site with Astro Starlight

generate_site.py writes the Starlight project in website/ and nothing
else: mkdocs.yml, the MkDocs hooks, theme overrides and stylesheet go,
and the wiki is written in Starlight's syntax. Both workflows set up
Node with the official actions/setup-node pinned by commit, run npm ci
from the committed lockfile, build with the strict link validator and
check the result with validate_site.py.

The search weighs a page by what it answers: Start here pages first,
a system page's cards and file names before a guide citing them, deep
guides, launchers, aliases and frozen copies after. Status badges,
hash blocks and the export change lists leave the index. The ranking
sits at the centre of a range where the 13 queries of search_gate.py
all answer, in the search box itself.

839 pages, every URL and 45,611 ids match the MkDocs build. The tests
read the Starlight content and syntax.

scripts/validate_schemas.py

@@ -439,11 +439,11 @@ def main() -> int:
         if release_path.exists():
             errors.extend(_validate_json_files([release_path], "release.schema.json"))
 
-        stats_path = ROOT / "docs" / "stats.json"
+        stats_path = ROOT / "website" / "public" / "stats.json"
         if stats_path.exists():
             errors.extend(_validate_json_files([stats_path], "stats.schema.json"))
 
-        api_dir = ROOT / "docs" / "api" / "v1"
+        api_dir = ROOT / "website" / "public" / "api" / "v1"
         api_database = api_dir / "database.json"
         if api_database.exists():
             errors.extend(