Docs: Enable gitlab-docs lint

What does this MR do?

• Reverts https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/23665
• Adds back docs linter to CE's and EE's .gitlab-ci.yml
• Closes https://gitlab.com/gitlab-com/gitlab-docs/issues/305

What are the relevant issue numbers?

Last Friday we were hit by https://gitlab.com/gitlab-org/gitlab-ce/issues/55038 and had to disable the docs lint for CE and EE: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/23665.

We solved the issue by not building the global nav in all projects except gitlab-docs: https://gitlab.com/gitlab-com/gitlab-docs/merge_requests/386.

This MR adds back the linter to CE/EE, closing https://gitlab.com/gitlab-com/gitlab-docs/issues/305.

Reviewer

• REVIEW APP: http://review-docs-add-d-r761ac.178.62.207.141.xip.io/ce/README.html
• Merge EE-MR first: https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/8803

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides
• Link to e2e tests MR added if this MR has Requires e2e tests label. See the Test Planning Process.
• Security reports checked/validated by reviewer