docs: move website ADR + plan into a dedicated docs/website/ namespace

The website subproject drew ADR numbers from the same global integer pool
as main, so every merge risked a collision — this already happened twice
(drafted 0042, bumped to 0044, both landing on numbers main had taken; main
has now used 0044 for relationship visualization). Give the website its own
ADR namespace so the two never compete again.

- docs/adr/0044-public-website-...md
    -> docs/website/adr/20260604-adr-website-001.md  (id: ADR-website-001)
- docs/plans/20260604-adr-0044-website.md
    -> docs/website/plans/20260604-website-implementation-plan.md
- new docs/website/adr/README.md index (dated <date>-adr-website-<NNN> seq)
- docs/adr/README.md: drop the 0044 entry, add a namespace pointer in the
  intro (keeps the list tail == merge-base, so main's 0044 merges cleanly)
- ADR-0000: record the subproject-ADR-namespace convention
- update references in STYLE.md, astro.config.mjs, the plan body

Handoff files left untouched as point-in-time history.

website/astro.config.mjs

@@ -27,7 +27,7 @@ export default defineConfig({
 			customCss: ['./src/styles/global.css'],
 			// Register the simple-mode command grammar with Expressive Code (Shiki).
 			expressiveCode: { shiki: { langs: [rdbmsLang, rdbmsSyntax] } },
-			// Pragmatic structure (ADR-0044 §7 / website/STYLE.md): Getting
+			// Pragmatic structure (ADR-website-001 §7 / website/STYLE.md): Getting
 			// started, Guides, Reference, Concepts. Autogenerated per directory;
 			// in-section order is controlled by each page's `sidebar.order`
 			// frontmatter.