feat(docs): publish llms.txt for AI agent consumption

Add the starlight-llms-txt plugin so the docs build emits llms.txt,
llms-full.txt, and llms-small.txt alongside the regular pages, making
the manual queryable by MCP-compatible agents without depending on a
third-party indexing service.

Also bump js-yaml, nanoid, and sharp in docs/site to clear three
high-severity advisories (quadratic YAML omap parsing, a zero-size
nanoid loop, and inherited libvips CVEs) flagged by npm audit while
touching the same lockfile.

docs/site/package.json

@@ -13,7 +13,8 @@
     "@astrojs/starlight": "^0.41.4",
     "@catppuccin/starlight": "^2.1.0",
     "astro": "^7.0.2",
-    "sharp": "^0.34.5",
-    "starlight-links-validator": "^0.25.2"
+    "sharp": "^0.35.3",
+    "starlight-links-validator": "^0.25.2",
+    "starlight-llms-txt": "^0.11.0"
   }
 }