Add a description to design uploads
### Release notes
Currently the [Design uploads](https://docs.gitlab.com/ee/user/project/issues/design_management.html#add-a-design-to-an-issue) have no metadata to explain their purpose, or why they are being uploaded. We've added a text field as a **description** to let users understand the image better.

## Summary
Currently the **Design uploads** :art: have no metadata on them explain what their purpose is, or why they are being uploaded. Adding a long text field as a **description** lets users understand the image better. Right now it's a very difficult process, users usually have to go and read entire comments just to do that.

## Proposal
In this epic, we will create a description for uploaded designs.
## UX Design
https://gitlab.com/gitlab-org/gitlab/-/issues/377546
epic