feat(fileuploader): Add documentation for file uploader

What does this MR do?

This adds File uploader definition, usage guidelines, and do's and dont's. The documentation describes how we use File uploaders in Pajamas. As a common component, it's important to add this documentation as a starting point.

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

Links

/label Pajamas