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/make_site_icons.py

@@ -11,8 +11,8 @@ four pins per side and the pixel glyph on a 4 by 4 grid. Three variants:
   shapes rasterised by a headless Chromium through Playwright, for browsers
   that take no SVG icon and for home-screen shortcuts.
 
-The files land in ``docs_assets/overrides/assets/brand/``, which MkDocs copies
-with the theme. The site build never runs this script.
+The files land in ``docs_assets/brand/``, which generate_site.py copies into
+the site. The site build never runs this script.
 
     python scripts/make_site_icons.py
 """
@@ -22,7 +22,7 @@ from __future__ import annotations
 from pathlib import Path
 
 ROOT = Path(__file__).resolve().parent.parent
-OUT = ROOT / "docs_assets" / "overrides" / "assets" / "brand"
+OUT = ROOT / "docs_assets" / "brand"
 
 HEADER = "#3b3f7a"
 GLYPH = "#8be0f7"