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/css/style.css

@@ -0,0 +1,116 @@
+/* theta42 docs site — shares the in-app dark navbar/footer + card look
+   (Bootstrap 5 + Font Awesome, same as the running apps) rather than a
+   generic Jekyll theme. */
+
+body {
+	background-color: #f4f5f6;
+}
+
+.navbar-brand img {
+	filter: drop-shadow(0 0 2px rgba(0, 0, 0, .4));
+}
+
+.navbar-nav .nav-link.active {
+	color: #fff;
+	font-weight: 600;
+}
+
+/* Markdown content typography, scoped to the card body so it doesn't leak
+   into the nav/footer. */
+.site-content h1:first-child {
+	margin-top: 0;
+}
+
+.site-content h1,
+.site-content h2,
+.site-content h3 {
+	font-weight: 700;
+}
+
+.site-content h2 {
+	margin-top: 2.5rem;
+	padding-bottom: .4rem;
+	border-bottom: 1px solid #e9ecef;
+}
+
+.site-content h3 {
+	margin-top: 1.75rem;
+}
+
+.site-content a {
+	color: #a3671f;
+	text-decoration-color: rgba(163, 103, 31, .35);
+}
+
+.site-content a:hover {
+	color: #8a5a16;
+}
+
+.site-content pre {
+	background-color: #212529;
+	color: #f8f9fa;
+	padding: 1rem 1.25rem;
+	border-radius: .375rem;
+	overflow-x: auto;
+}
+
+.site-content code {
+	color: #a3671f;
+	background-color: #f4f0e8;
+	padding: .15em .4em;
+	border-radius: .25rem;
+	font-size: .875em;
+}
+
+.site-content pre code {
+	color: inherit;
+	background: none;
+	padding: 0;
+}
+
+.site-content table {
+	display: block;
+	overflow-x: auto;
+	width: 100%;
+	border-collapse: collapse;
+	margin: 1.25rem 0;
+}
+
+.site-content table th,
+.site-content table td {
+	border: 1px solid #dee2e6;
+	padding: .5rem .75rem;
+	text-align: left;
+}
+
+.site-content table th {
+	background-color: #f8f9fa;
+}
+
+.site-content blockquote {
+	border-left: 4px solid #C59341;
+	padding: .5rem 1rem;
+	margin: 1.25rem 0;
+	background-color: #f8f6f1;
+	color: #495057;
+}
+
+.site-content img {
+	max-width: 100%;
+	height: auto;
+}
+
+/* Screenshot grids in the markdown use width="49%" inline attrs for a
+   two-up desktop layout -- stack them on narrow screens instead of
+   squeezing to illegibility. */
+@media (max-width: 576px) {
+	.site-content img[width] {
+		width: 100% !important;
+		margin-bottom: .75rem;
+	}
+}
+
+.site-content hr {
+	margin: 2rem 0;
+	border-top: 1px solid #e9ecef;
+}