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>

README.md

@@ -19,11 +19,11 @@ phone-home, no hosted control plane, and no per-user pricing.
 
 | Dashboard | Users |
 | --- | --- |
-| ![Dashboard](docs/images/dashboard.png) | ![User list](docs/images/users.png) |
+| [![Dashboard](docs/images/dashboard.png)](docs/images/dashboard.png) | [![User list](docs/images/users.png)](docs/images/users.png) |
 
 | Groups | OAuth Apps |
 | --- | --- |
-| ![Groups](docs/images/groups.png) | ![OAuth clients](docs/images/oauth-clients.png) |
+| [![Groups](docs/images/groups.png)](docs/images/groups.png) | [![OAuth clients](docs/images/oauth-clients.png)](docs/images/oauth-clients.png) |
 
 ## Features