Move Bugs, Performance Improvements, and Usability Improvements to a new section in the Release Post

Why is this change being made?

To allow us to display additional release post items below secondary features in a single column above deprecations.

#11214 (comment 564565592)

• Moves the 13.11 Bugs, Performance Improvements, and UX Improvements from secondary to tertiary format
• Updates the sample files for Bugs, Performance Improvements, and UX Improvements to tertiary format

Review:

• 13.11 https://pcalder-release-post-tertiary.about.gitlab-review.app/releases/2021/04/22/gitlab-13-11-released/#bug-fixes
• 13.10 https://about.gitlab.com/releases/2021/03/22/gitlab-13-10-released/#bug-fixes (no change, these entries still use the secondary format but could be updated anytime)

TODO before merging:

• Confirm we do not need the available_in SaaS / Self Managed buttons and labels => !81042 (comment 564969040)
• Confirm ordering of these three items (currently alphabetical by title: Bugs, Perf.., Usability..) => !81042 (comment 564969040)
• Test validate and generate release post items (should not affect theses)
• DRI approval to merge => !81042 (comment 566341151)

Follow up:

• Confirmed: we do not need to migrate previous releases (this MR migrates 13.11 only) => !81042 (comment 566341151)
• Update handbook - see also related MR: !81235 (merged)
• Update Release Post MR template
• Ensure the correct sample files are used when generating the 13.12 YAML files

Author Checklist

• Provided a concise title for the MR
• Added a description to this MR explaining the reasons for the proposed change, per say-why-not-just-what
• Assign this change to the correct 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 in 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 relate to any part of the project other than updates to content and/or data files please make sure to ping @gl-static-site-editor in a comment for a review and merge. For example changes to .gitlab-ci.yml, JavaScript/CSS/Ruby code or the layout files. (this requirement has been removed pending identification of a new DRI for the handbook)