Rename and document firstKnock option for HTTP signature spec

Rename FederationOptions.defaultHttpMessageSignaturesSpec to firstKnock
and add comprehensive documentation for the double-knocking mechanism
when communicating with unknown servers.

- Rename defaultHttpMessageSignaturesSpec to firstKnock throughout codebase
- Add JSDoc documentation to FederationOptions.firstKnock
- Document firstKnock option in docs/manual/federation.md
- Update CHANGES.md with detailed description of the feature

Co-Authored-By: Claude <noreply@anthropic.com>