docs: update README, about resource, repo metadata for OWASP+NIST unified server

- README: full rewrite reflecting 31 tools, 17 data sources, 3230+ records
- server.py about() resource: 24 tools → 31 tools, NIST section added
- Repo description: unified OWASP+NIST MCP server
- Repo homepage: security-framework docs
- 15 topics: mcp, owasp, nist, cybersecurity, compliance, etc.

Author: zer0-kr