feat(routing): redirect section landings + harden 404 / no directory listings

- astro.config.mjs: section-only URLs now redirect to the first article in
  that section. /whp/admin/ used to render Apache's directory listing
  because no index.html existed; now it serves a meta-refresh to
  /whp/admin/overview/. Same for /whp/, /whp/getting-started/,
  /whp/how-to/, /whp/site-builder/, /whp/reference/, /whp/add-ons/.
- public/.htaccess: ships in dist, disables Options Indexes + MultiViews
  (defense in depth so any future section without a redirect doesn't
  leak a listing), and routes 404/403 to /404.html.
- src/content/docs/404.md: replaces the bare 'check the URL' tagline
  with two explicit actions — 'Go to the knowledge base home' and
  'WHP getting started'.

src/content/docs/404.md

@@ -0,0 +1,17 @@
+---
+title: Page not found
+description: That URL doesn't exist on our knowledge base.
+template: splash
+editUrl: false
+hero:
+  title: '404'
+  tagline: That URL doesn't exist on our knowledge base.
+  actions:
+    - text: Go to the knowledge base home
+      link: /
+      icon: right-arrow
+      variant: primary
+    - text: WHP getting started
+      link: /whp/getting-started/welcome/
+      variant: secondary
+---