Re-enable haml-lint in docs pipelines

What does this MR do and why?

With the completion of #419673 (closed), I was supposed to re-enable haml-lint in docs pipelines, but forgot, which caused: gitlab-org/quality/engineering-productivity/master-broken-incidents#8780 (closed)

The broken master is already resolved, but this will ensure it doesn't happen again.

Should be safe to enable immediately, testing locally on latest master:

❯ bundle exec haml-lint -i DocumentationLinks

2197 files inspected, 0 lints detected

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