Add docs-lint markdown rule to disallow GitLab Flavored Markdown emojis in docs

Proposal

I've noticed a few MRs merged into master that used GitLab Flavored Markdown emojis such as ✅ (:white_check_mark:) and ❌ (:x:) which are not supported in GitLab docs. This caused the markdown to render incorrectly and needed to be fixed in follow-up MRs:

• Fix icons in DS docs (!87355 - merged)
• Fix invalid markdown in Secure Team standards a... (!93096 - merged)

The purpose of this issue is to add a docs-lint markdown rule to prevent these unsupported emojis from being added to GitLab docs.

/cc @rdickenson