Move community extensions table from README to docs site (#2560)

* Add Agent Governance extension to community catalog

Add agent-governance extension submitted by @bigsmartben to:
- extensions/catalog.community.json (alphabetical order)
- README.md community extensions table

Closes #2552

* Move community extensions table from README to docs site

- Create docs/community/extensions.md with full extensions table
- Replace ~120-line table in README.md with summary + link to docs site
- Add Extensions entry to docs/toc.yml under Community
- Update add-community-extension SKILL.md references