docs: add Nix installation guide and navigation

- Expand docs/nix.md from runtime-only to full installation guide
- Reference nix-clawdbot as the recommended Nix setup path
- Add "Installation" section to docs.json navigation (wizard, nix, docker, setup)
- Add Nix link to README quick links

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

docs/docs.json

@@ -27,11 +27,19 @@
         "pages": [
           "index",
           "onboarding",
-          "wizard",
           "clawd",
           "faq"
         ]
       },
+      {
+        "group": "Installation",
+        "pages": [
+          "wizard",
+          "nix",
+          "docker",
+          "setup"
+        ]
+      },
       {
         "group": "Core Concepts",
         "pages": [