Rename "Integrating skills" guide to "Adding skills support"

Addresses reviewer feedback that "Integrating" could be misread as
integrating skills into a workflow rather than into a client. Renames
`integrating-skills.mdx` to `adding-skills-support.mdx` and updates all
references in `docs.json`, `home.mdx`, and `what-are-skills.mdx`.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

docs/docs.json

@@ -28,7 +28,7 @@
       {
         "group": "For client implementors",
         "pages": [
-          "client-implementation/integrating-skills"
+          "client-implementation/adding-skills-support"
         ]
       }
     ]
@@ -48,7 +48,7 @@
   "redirects": [
     {
       "source": "/integrate-skills",
-      "destination": "/client-implementation/integrating-skills"
+      "destination": "/client-implementation/adding-skills-support"
     }
   ]
 }