Update docs ee/development/uploads

This MR updates ee/development/uploads documentation and includes a related link as an example. When I tried to implement API endpoint for uploading a file, it was hard to understand GitLab's approach for doing this without examples until I found https://docs.gitlab.com/ee/development/packages.html#file-uploads. It would be cool to include this link in the general page for file uploads.

This MR also fixes sequence diagrams, which are currently either not displayed or displaying their source code: