Explain how to update a fork

What does this MR do?

Updates https://docs.gitlab.com/ee/user/project/repository/forking_workflow.html to provide clearer instructions on the methods for updating your fork.

How did we not have this already?! 🙈

Note to self: https://docs.gitlab.com/ee/gitlab-basics/start-using-git.html may also need some changes to make sure we're adhering to SSOT.

Related issues

• Related to Docs Hackathon Jan 2023 planning issue (technical-writing#740 - closed) because my teammates need this info ASAP
• Related to Fetch new upstream contents when fork is behind (#330243 - closed) because we'll likely want to build more info into this page

Related resources that are not issues:

• https://docs.gitlab.com/ee/development/documentation/workflow.html#how-to-update-the-docs may need an update afterward, or we may need to incorporate some of its info here
• https://forum.gitlab.com/t/refreshing-a-fork/32469/2
• https://about.gitlab.com/blog/2016/12/01/how-to-keep-your-fork-up-to-date-with-its-origin/

Author's checklist

• Optional. Consider taking the GitLab Technical Writing Fundamentals course.
• Follow the:
  • Documentation process.
  • Documentation guidelines.
  • Style Guide.
• If you're adding or changing the main heading of the page (H1), ensure that the product tier badge is added.
• If you are a GitLab team member, request a review based on:
  • The documentation page's metadata.
  • The associated Technical Writer.

If you are a GitLab team member and only adding documentation, do not add any of the following labels:

• ~"frontend"
• ~"backend"
• ~"type::bug"
• ~"database"

These labels cause the MR to be added to code verification QA issues.

Reviewer's checklist

Documentation-related MRs should be reviewed by a Technical Writer for a non-blocking review, based on Documentation Guidelines and the Style Guide.

If you aren't sure which tech writer to ask, use roulette or ask in the #docs Slack channel.

• If the content requires it, ensure the information is reviewed by a subject matter expert.
• Technical writer review items:
  • Ensure docs metadata is present and up-to-date.
  • Ensure the appropriate labels are added to this MR.
  • Ensure a release milestone is set.
  • If relevant to this MR, ensure content topic type principles are in use, including:
    • The headings should be something you'd do a Google search for. Instead of Default behavior, say something like Default behavior when you close an issue.
    • The headings (other than the page title) should be active. Instead of Configuring GDK, say something like Configure GDK.
    • Any task steps should be written as a numbered list.
    • If the content still needs to be edited for topic types, you can create a follow-up issue with the docs-technical-debt label.
• Review by assigned maintainer, who can always request/require the reviews above. Maintainer's review can occur before or after a technical writer review.