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

The GitHub Pages site used the generic jekyll-theme-cayman theme --
purple gradient hero, no site nav, no per-page SEO. Replaced with a
custom layout that mirrors 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
(copyright, MIT license, GitHub, Changelog links).

- New cross-page nav (Home/Deployment/Configuration/OAuth/LDAP/
  Changelog) -- there was previously no way to get from one docs
  page to another except a single "Back to Home" link per page.
- SEO: jekyll-seo-tag + jekyll-sitemap (both GitHub-Pages-supported
  plugins, no custom build needed) -- real per-page meta description,
  Open Graph/Twitter card tags, canonical URLs, JSON-LD, sitemap.xml,
  and a robots.txt referencing it. Added a real description to every
  page's front matter (none existed before).
- Mobile: Bootstrap's responsive grid + collapsible navbar; the
  screenshot pairs in index.md (inline width="49%" for a two-up
  desktop layout) now stack to full-width below 576px instead of
  squeezing illegibly small.

Verified with a real Jekyll build (jekyll/jekyll Docker image, no
Ruby available locally) + Playwright: desktop and mobile (375px)
screenshots of the home and deployment pages, mobile nav toggle
open/close, active-link highlighting per page, zero console/page
errors, and confirmed real SEO output (meta description, OG/Twitter
tags, canonical, JSON-LD, sitemap.xml, robots.txt) via curl against
the served site.

docs/_config.yml

@@ -1,9 +1,44 @@
 title: SSO Manager
-description: A self-hosted OpenID Connect provider with an OpenLDAP directory and a web management UI
-theme: jekyll-theme-cayman
-show_downloads: false
+description: A self-hosted OpenID Connect provider with a bundled OpenLDAP directory and a web management UI, for home labs and small businesses that want their own identity provider.
+url: "https://theta42.github.io"
+baseurl: "/sso-manager-node"
+logo: /assets/img/theta42.svg
+lang: en_US
+
+plugins:
+  - jekyll-seo-tag
+  - jekyll-sitemap
+
 github:
   repository_url: https://github.com/theta42/sso-manager-node
   zip_url: https://github.com/theta42/sso-manager-node/archive/refs/heads/master.zip
   tar_url: https://github.com/theta42/sso-manager-node/archive/refs/heads/master.tar.gz
-  repository_name: theta42/sso-manager-node
+  repository_name: theta42/sso-manager-node
+
+nav:
+  - title: Home
+    page: /
+    icon: fa-house
+  - title: Deployment
+    page: /deployment.html
+    icon: fa-server
+  - title: Configuration
+    page: /configuration.html
+    icon: fa-gears
+  - title: OAuth
+    page: /oauth.html
+    icon: fa-key
+  - title: LDAP
+    page: /ldap.html
+    icon: fa-address-book
+  - title: Changelog
+    url: https://github.com/theta42/sso-manager-node/blob/master/CHANGELOG.md
+    icon: fa-list
+
+defaults:
+  - scope:
+      path: ""
+      type: "pages"
+    values:
+      layout: default
+      image: /assets/img/theta42.svg