Update docs for GitLab UI integration testing

What does this MR do?

Update docs for GitLab UI integration testing

MR adds more information about creating a custom GitLab UI build package and testing it on the GitLab forked mirror. Changes were made to help developers working on both GitLab UI and the core product make changes confidently and with fewer late test breaks.

MR closes #3041 (closed)

Does this MR meet the acceptance criteria?

This checklist encourages the authors, reviewers, and maintainers of merge requests (MRs) to confirm changes were analyzed for conformity with the project's guidelines, security and accessibility.

Toggle the acceptance checklist

Conformity

• The “What does this MR do?” section in the MR description is filled out, explaining the reasons for and scope of the proposed changes, per “Say why not just what”.
  • For example, if the MR is focused on usage guidelines, addressing accessibility challenges could be added in a separate MR.
• Relevant label(s) are applied to the MR.
• The MR is added to a milestone.
• Added the ~"component:*" label(s) if applicable.
• A changeset is added when this MR will do a patch, minor, or major update. More information can be found in doc/publishing-packages.md.

Documentation

• If creating a new component page from scratch, it follows the page template structure.
• Content follows the Pajamas voice and tone guidelines, falling back on the GitLab Documentation Style Guide when needed.
• Related pages are cross-linked, where helpful. Component pages have related components and patterns defined in their Markdown front matter.
• If embedding a Figma file, it follows the Figma embed guide.
• Review requested from any GitLab designer or directly from a maintainer or trainee maintainer.
• Maintainer review follows the Pajamas UX maintainer review checklist.