Document how to manually release a new version of code-server

[jsjoeio]

We ran into issues with our release flow the other week. Yet, we didn't have the proper documentation in place to trigger a manual release.

• Document manual release flow in docs/MAINTAINERS_HANDBOOK.md
• Ensure all maintainers (and one backup maintainer/Coder employee) have proper rights and access tokens for publishing on registries (npm/Docker)

Issue created as suggestion action from regression report

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no activity occurs in the next 5 days.