Release post: CI test for duplicate anchors

Proposal

Implement a CI test to check release posts for identical anchors.

Reasons

For example, in the 13.8 release post, we had 2 identical anchors:

  • #bug-fixes from the Bug fixes heading:

    Screen_Shot_2021-01-20_at_12.10.00

  • #bug-fixes from a subheading of the Runner section:

    Screen_Shot_2021-01-20_at_12.09.30

As the "bug fixes" on the Runner section was displayed first, the link to https://release-13-8.about.gitlab-review.app/releases/2021/01/22/gitlab-13-8-released/index.html#bug-fixes always led to the Runner section instead of the Bug fixes section.

cc/ @fseifoddini