docs: refactor README - reduce size, add crate READMEs

- Reduce main README from ~766 to ~169 lines (78% reduction)
- Add README for mcp-core crate (foundation types, traits, errors)
- Add README for mcp-files crate (virtual filesystem)
- Add README for mcp-introspector crate (MCP server analysis)
- Link to crate READMEs for detailed documentation

Author: bug-ops