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/index.md

@@ -1,6 +1,7 @@
 ---
 layout: default
 title: Home
+description: A self-hosted OpenID Connect provider with a bundled OpenLDAP directory and a web management UI. One login for your modern apps, one LDAP directory for the rest, no phone-home.
 ---
 
 # SSO Manager