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 running the two separately, what you get, a minimal
  "get it" snippet) instead of a full config/architecture reference --
  that content still lives in the repo (README, docs/*.md), linked from
  here.
- Cross-link to SSO Manager's and Proxy'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>

README.md

@@ -24,7 +24,7 @@ The SSO Manager and the proxy it fronts, both stood up by one `./setup.sh` run:
 
 | SSO Manager Dashboard | Proxy Hosts |
 | --- | --- |
-| ![SSO Manager dashboard](docs/images/sso-dashboard.png) | ![Proxy host list](docs/images/proxy-hosts.png) |
+| [![SSO Manager dashboard](docs/images/sso-dashboard.png)](docs/images/sso-dashboard.png) | [![Proxy host list](docs/images/proxy-hosts.png)](docs/images/proxy-hosts.png) |
 
 **Why use this instead of running the two separately?** The two only become
 useful once the proxy is registered as an OIDC client of the SSO and pointed at