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/concepts/memory.md

@@ -1,4 +1,5 @@
 ---
+title: "Memory"
 summary: "How OpenClaw memory works (workspace files + automatic memory flush)"
 read_when:
   - You want the memory file layout and workflow