docs: simplify security release process

@pslaughter used the original extension release process when designing the gitlab-web-ide release process and introduced several simplifications: docs: Add security_releases documentation (gitlab-web-ide!210 - merged)

This MR backports the simplifications to the extension project

Namely:

• we set the security fork to mirror the main project:
  • 
• the security release branch is synchronized with the main project via an MR from the security project to the main project

relates to #813 (closed)