Doc: add documentation for additional commit operations

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What does this MR do and why?

• Enable additionalSourceControlOperations feature flag
• Update user docuemntation for the new features
  • Commit and force push
  • Amend commit and force push

Related

1. Related Issue: gitlab-web-ide#482 (closed)
2. Related Merge Request: gitlab-web-ide!507 (merged)

Screenshots or screen recordings

Video	Description
force_push_demo.mov	This video demonstrates how the force push feature works in the Web IDE.
amend_commit_demo.mov	This video demonstrates how the amend & force push feature works in the Web IDE.

How to set up and validate locally

MR acceptance checklist

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