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 SSO Manager 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>

README.md

@@ -22,12 +22,12 @@ proxy serves them over TLS with auto-renewing certs and no downtime on changes.
 
 | Hosts | Authentication |
 | --- | --- |
-| ![Host list](docs/images/hosts.png) | ![Per-host SSO auth](docs/images/host-auth-sso.png) |
+| [![Host list](docs/images/hosts.png)](docs/images/hosts.png) | [![Per-host SSO auth](docs/images/host-auth-sso.png)](docs/images/host-auth-sso.png) |
 
 Basic auth and SSO are mutually exclusive per host, with per-user password
 management once basic auth is enabled:
 
-![Per-host basic auth](docs/images/host-auth-basic.png)
+[![Per-host basic auth](docs/images/host-auth-basic.png)](docs/images/host-auth-basic.png)
 
 ## Why this over the alternatives