Update documentation for compliance with CommonMark

What does this MR do?

Some of our documentation files will not render correctly when we upgrade from RedCarpet to CommonMark. This addresses the problem by making changes that are compatible with both RedCarpet and CommonMark. Necessary changes were determined by using the tool https://gitlab.com/digitalmoksha/diff_redcarpet_cmark

Currently docs.gitlab.com is generated using RedCarpet. The changes should allow for proper rendering on the doc site, well as when viewing the rendered markdown in the source repository (using CommonMark)

What are the relevant issue numbers?

https://gitlab.com/gitlab-org/gitlab-ce/issues/50695

Related CE MR: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/21385

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
• EE specific content should be in the top level /ee folder
• For a paid feature, have we considered GitLab.com plans, how it works for groups, and is there a design for promoting it to users who aren't on the correct plan?