docs: Update index with new navigation structure

why: The old nav (quickstart, about, topics, api) was flat and mixed
concerns. The new layered nav (Getting Started, Tools, Guides,
Reference, Project) matches the pattern used by top MCP servers.
what:
- Rewrite docs/index.md with 5-section toctree
- Remove docs/about.md (content distributed to concepts, architecture,
  tools/index, safety, and configuration)

Author: tony

docs/about.md

@@ -14,22 +14,50 @@ MCP server for tmux, powered by [libtmux](https://libtmux.git-pull.com/).
 ```
 
 ```{toctree}
+:caption: Getting Started
 :maxdepth: 2
 :hidden:
 
+installation
 quickstart
-about
-topics/index
-api/index
+clients
+concepts
+configuration
+safety
 ```
 
 ```{toctree}
-:caption: Project
+:caption: Tools
+:maxdepth: 2
 :hidden:
 
-developing
+tools/index
+```
+
+```{toctree}
+:caption: Guides
+:maxdepth: 2
+:hidden:
+
+guides/troubleshooting
+```
+
+```{toctree}
+:caption: Reference
+:maxdepth: 2
+:hidden:
+
+reference/api/index
+reference/compatibility
 history
+```
+
+```{toctree}
+:caption: Project
+:hidden:
+
+architecture
+development
 glossary
 GitHub <https://github.com/tmux-python/libtmux-mcp>
 ```
-