Redesign docs site: match the app's own look, add SEO, mobile-ready

Same treatment as the sso-manager-node companion PR: replaced the
generic jekyll-theme-cayman theme with a custom layout mirroring the
actual app UI -- dark fixed navbar with the theta42 logo, Bootstrap 5
+ Font Awesome (same stack the app uses), content in a card, dark
footer matching bottom.ejs. Keeps this repo's own favicon.svg
(confirmed genuinely distinct SVG artwork from the shared theta42
logo, not a duplicate) as the browser-tab icon.

- New cross-page nav (Home/Installation/Architecture/API/Docker/
  Contributing/Changelog).
- SEO: jekyll-seo-tag + jekyll-sitemap, per-page meta description,
  OG/Twitter card tags, canonical URLs, JSON-LD, sitemap.xml,
  robots.txt.
- Mobile: Bootstrap's responsive grid + collapsible navbar; the
  screenshot pairs in index.md stack to full-width below 576px.

Verified with a real Jekyll build (jekyll/jekyll Docker image) +
Playwright: desktop and mobile (375px) screenshots, mobile nav
toggle, active-link highlighting, zero console/page errors, and
confirmed real SEO output + the correct (non-shared) favicon via curl
against the served site.

docs/_config.yml

@@ -1,9 +1,47 @@
 title: Proxy
-description: A reverse proxy and HTTPS termination service using OpenResty/nginx with a management API and web GUI
-theme: jekyll-theme-cayman
-show_downloads: false
+description: A reverse proxy and HTTPS termination service built on OpenResty/nginx, with an OIDC + LDAP-aware management API and web GUI.
+url: "https://theta42.github.io"
+baseurl: "/proxy"
+logo: /assets/img/theta42.svg
+lang: en_US
+
+plugins:
+  - jekyll-seo-tag
+  - jekyll-sitemap
+
 github:
   repository_url: https://github.com/theta42/proxy
   zip_url: https://github.com/theta42/proxy/archive/refs/heads/master.zip
   tar_url: https://github.com/theta42/proxy/archive/refs/heads/master.tar.gz
   repository_name: theta42/proxy
+
+nav:
+  - title: Home
+    page: /
+    icon: fa-house
+  - title: Installation
+    page: /installation.html
+    icon: fa-download
+  - title: Architecture
+    page: /architecture.html
+    icon: fa-sitemap
+  - title: API
+    page: /api.html
+    icon: fa-code
+  - title: Docker
+    page: /docker.html
+    icon: fa-box
+  - title: Contributing
+    page: /contributing.html
+    icon: fa-code-branch
+  - title: Changelog
+    url: https://github.com/theta42/proxy/blob/master/CHANGELOG.md
+    icon: fa-list
+
+defaults:
+  - scope:
+      path: ""
+      type: "pages"
+    values:
+      layout: default
+      image: /assets/img/theta42.svg