Add File uploads docs to side nav

The following discussion from gitlab!80392 (merged) should be addressed:

• @msedlakjakubowski started a discussion: (+2 comments)

  @mkaeppler We'll need to add the new pages to the docs nav when this is merged: https://gitlab.com/gitlab-org/gitlab-docs/-/blob/main/content/_data/navigation.yaml.

  Can you open an MR with suggested placement later?