Turn GitHub Pages into a marketing landing page; cross-link, drop download buttons

- Rewrite docs/index.md as a short landing page (what it is, screenshots,
  why this over the alternatives, features, a minimal "get it" snippet)
  instead of a full documentation dump — full docs live in the repo
  (README, docs/*.md) and are linked from here.
- Cross-link to Proxy and theta-env's own Pages sites.
- Screenshots are now clickable (open full size) on both the Pages site
  and the README.
- Disable show_downloads in docs/_config.yml — the Cayman theme's
  "Download .zip/.tar.gz" buttons are gone; "View on GitHub" (which links
  back to the repo) is the only header link now.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

docs/_config.yml

@@ -1,7 +1,7 @@
 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: true
+show_downloads: false
 github:
   repository_url: https://github.com/theta42/sso-manager-node
   zip_url: https://github.com/theta42/sso-manager-node/archive/refs/heads/master.zip