docs: reshuffle AI section and remove Agent Rules

- Group AI nav as Rocky AI + Agent Resources (Overview, MCP Server, Skills, Markdown Access, llms.txt)
- Remove the Agent Rules page and all internal links; redirect /ai/rules to /ai/overview
- Clarify Skills (CLI-first, repo access) vs MCP (desktop/sandboxed) on the overview
- De-duplicate Markdown Access and llms.txt content from the overview, link to the standalone pages
- Move the MCP/Skills/CLI comparison table to the overview; shorten and link from the MCP server page

Author: stefanjudis