Update user markdown documentation for CommonMark

What does this MR do?

Updates the user markdown documentation to indicate we now use CommonMark for repository files.

What are the relevant issue numbers?

https://gitlab.com/gitlab-org/gitlab-ce/issues/43011

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?

Related CE Merge Request

https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/21509