Update contribution guidelines in doc template

What does this MR do?

We've "learned some lessons" during the last hackathon, so I'm updating the docs-cleanup template to try and reduce the amount of issues that during the hectic time of a hackathon.

• Link to MR guidelines for contributors. The key is that the first few steps make it clear that you should create a feature branch, and not work from the default branch.
• Link to commit message guidelines. We often have problems with contributions with commit messages that fail pipelines. We can work around it, but sometimes it's tricky and it's best to not need to work around it at all.
• Explain that we normally expect one MR per issue.
• Explain that they should use the Documentation template and fill it out, to avoid the plentiful MRs we get with absolutely no details.
• General copy edits.

Related issues

Resolves technical-writing#664 (closed)

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 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.