Create update a fork page

What does this MR do and why?

In this MR, a new page for Update a fork is added to the Use Git --> Basic operations section.

The information on this page is Git-related info from Update a fork from the command line and Push to a fork as an upstream member.

It is part of a larger issue to reorganize Git documentation to improve the user experience by separating GitLab UI and Git CLI tasks.

Navigation

- Basic operations
  - Create a project
  - Clone a repository
  - Create a branch
  - Stage, commit, and push changes
  - Stash changes
  - Add files to your branch
  - Merge your branch
  - Update a fork

Review app

• NEW Update a fork

Related issues

• Issue: #499459
• Child item: #499466 (closed)
• MR to update navigation: Add nav item for new Update a fork page (gitlab-docs!5162 - merged)