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/artifacts.py

@@ -219,7 +219,7 @@ def hold_artifact_lock(directory: str) -> Iterator[None]:
 
 
 # What a pipeline run writes at fixed paths, whatever its --output-dir.
-REPOSITORY_OUTPUTS = ("database.json", "install/", "README.md", "CONTRIBUTING.md", "docs/")
+REPOSITORY_OUTPUTS = ("database.json", "install/", "README.md", "CONTRIBUTING.md", "website/")
 
 
 @contextlib.contextmanager