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

Same treatment as the proxy and sso-manager-node companion PRs. This
repo has no app UI of its own (it's a bash orchestrator), so both the
nav logo and favicon use the shared theta42.svg mark -- matching the
family look shown in its own screenshots (the SSO Manager/proxy
dashboards it stands up).

- New cross-page nav (Home/Quickstart/Architecture/Standalone/
  Changelog) -- replaces index.md's old "More docs" section, now
  redundant with the top nav.
- 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 pair in index.md stacks to full-width below 576px.
- Added docs/_site to .gitignore (missing entirely before -- the
  other two repos already had it).

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 (meta description, OG/Twitter tags,
canonical, JSON-LD, sitemap.xml, robots.txt) via curl against the
served site.

docs/architecture.md

@@ -1,6 +1,7 @@
 ---
 layout: default
 title: Architecture
+description: How theta-env composes the SSO Manager and proxy submodules — the OIDC/LDAP wiring setup.sh generates from one domain.
 ---
 
 # Architecture