Update file uploader with markdown file uploader

What does this MR do?

Follow up from #595 (closed) to add documentation for markdown file uploader.

Does this MR meet the acceptance criteria?

• Add a description to this MR explaining the reasons for the proposed change, per say-why-not-just-what
• Include MR scope that reflects the purpose of the change in the description
  • For example, if this MR is focused on usage guidelines, addressing accessibility challenges could be added separately with a different MR
• If creating a new page on Pajamas from scratch, follow the page template structure
• Follow the Pajamas Voice & Tone Guidelines and GitLab Documentation Style Guide
• Link components to and from other related components and patterns on the .md file, where helpful
• Assign it to a reviewer. All GitLab designers are reviewers, but you can also assign it directly to a maintainer or trainee maintainer
• Apply the correct component label(s)
• Select a milestone or due date

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