Add docs feedback and improvements section

Why is this change being made?

Takes the backlog discussion in gitlab-org/technical-writing#646 (comment 1072372440), and documents labels for docs feedback and improvement issues, with the goal in mind that this will help us:

1. Clean up our existing backlog.
2. Triage and prioritize new non-feature/milestone related issues.

Context: As part of our initial analysis on docs backlog items we found some common patterns, and identified labels that could help us make the work more visible and more easily transferrable as group assignments change.

MVC: This is an MVC update to the handbook to align on the labels to use. Next steps could include automation and improvement of the template, clean-up of the backlog, and applying the labels to docs-only issues going forward. In a separate MR, the Disqus section in the handbook can probably be moved up to be a sub-section of this new heading.

Author Checklist

• Provided a concise title for this Merge Request (MR)
• Added a description to this MR explaining the reasons for the proposed change, per say why, not just what
  • Copy/paste the Slack conversation to document it for later, or upload screenshots. Verify that no confidential data is added.
• Assign reviewers for this MR to the correct Directly Responsible Individual/s (DRI)
  • If the DRI for the page/s being updated isn’t immediately clear, then assign it to one of the people listed in the Maintained by section on the page being edited
  • If your manager does not have merge rights, please ask someone to merge it AFTER it has been approved by your manager in #mr-buddies
• If the changes affect team members, or warrant an announcement in another way, please consider posting an update in #whats-happening-at-gitlab linking to this MR
  • If this is a change that directly impacts the majority of global team members, it should be a candidate for #company-fyi. Please work with internal communications and check the handbook for examples.

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