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'.

astro.config.mjs

@@ -7,6 +7,18 @@ import { fileURLToPath } from 'node:url';
 // https://astro.build/config
 export default defineConfig({
   site: 'https://kb.anhonesthost.com',
+  // Send section-only URLs to the first article in that section. Without these,
+  // hitting e.g. /whp/admin/ would render the Apache directory listing because
+  // there is no index.html in that folder.
+  redirects: {
+    '/whp/': '/whp/getting-started/welcome/',
+    '/whp/getting-started/': '/whp/getting-started/welcome/',
+    '/whp/how-to/': '/whp/how-to/add-a-domain/',
+    '/whp/site-builder/': '/whp/site-builder/overview/',
+    '/whp/reference/': '/whp/reference/service-hostnames/',
+    '/whp/add-ons/': '/whp/add-ons/overview/',
+    '/whp/admin/': '/whp/admin/overview/',
+  },
   vite: {
     resolve: {
       alias: {