Handle unowned feature categories in categories.yml

Why is this change being made?

This MR removes the feature category snippets from categories.yml because it was demoted in Demote snippets from Editor category to feature... (!108078 - merged). See also !108078 (diffs, comment 1260803774).

This MR also adds the unowned feature category gitlab_docs to Manage Foundations because of https://about.gitlab.com/direction/manage/foundations/gitlab_docs/.

Contributes to #14365 and #14365 (comment 1260902499).

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
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