docs: SEO and AI discoverability improvements

- Add description to docs.json for llms.txt blockquote summary
- Add title frontmatter to 10 docs files for llms.txt link text
- ci(docker): skip builds for docs-only changes

docs/docs.json

@@ -1,6 +1,7 @@
 {
   "$schema": "https://mintlify.com/docs.json",
   "name": "OpenClaw",
+  "description": "Self-hosted gateway that connects WhatsApp, Telegram, Discord, iMessage, and more to AI coding agents. Run a single Gateway process on your own machine and message your AI assistant from anywhere.",
   "theme": "mint",
   "icons": {
     "library": "lucide"