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/assets/img/theta42.svg

@@ -0,0 +1,51 @@
+<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 400 400" width="100%" height="100%">
+  <defs>
+    <linearGradient id="gold-grad" x1="0%" y1="0%" x2="100%" y2="100%">
+      <stop offset="0%" stop-color="#C59341" />
+      <stop offset="20%" stop-color="#E4B869" />
+      <stop offset="40%" stop-color="#FBF0B9" />
+      <stop offset="60%" stop-color="#DFB260" />
+      <stop offset="80%" stop-color="#BC8837" />
+      <stop offset="100%" stop-color="#A36F28" />
+    </linearGradient>
+
+    <linearGradient id="text-grad" x1="0%" y1="100%" x2="100%" y2="0%">
+      <stop offset="0%" stop-color="#FFFFFF" />
+      <stop offset="40%" stop-color="#F5E3B5" />
+      <stop offset="70%" stop-color="#D4A343" />
+      <stop offset="100%" stop-color="#8A5A16" />
+    </linearGradient>
+
+    <filter id="drop-shadow" x="-20%" y="-20%" width="140%" height="140%">
+      <feDropShadow dx="0" dy="8" stdDeviation="6" flood-color="#000000" flood-opacity="0.4"/>
+    </filter>
+  </defs>
+
+  <g filter="url(#drop-shadow)">
+    <g fill="url(#gold-grad)">
+      <path d="M 200,40 
+               C 290,40 350,110 350,200 
+               C 350,290 290,360 200,360 
+               C 110,360 50,290 50,200 
+               C 50,110 110,40 200,40 Z 
+               M 200,75 
+               C 130,75 88,130 88,200 
+               C 88,270 130,325 200,325 
+               C 270,325 312,270 312,200 
+               C 312,130 270,75 200,75 Z" 
+            fill-rule="evenodd" />
+      
+      <path d="M 88,190 L 140,190 C 140,190 142,210 140,210 L 88,210 Z" />
+      
+      <path d="M 260,190 L 312,190 C 312,190 310,210 260,210 Z" />
+    </g>
+
+    <text x="200" y="222" 
+          font-family="system-ui, -apple-system, 'Segoe UI', Roboto, Helvetica, Arial, sans-serif" 
+          font-size="78" 
+          font-weight="900" 
+          fill="url(#text-grad)" 
+          text-anchor="middle"
+          letter-spacing="-2">42</text>
+  </g>
+</svg>