GitLab Version Check - Document UI

What / Why

Existing documentation: https://about.gitlab.com/handbook/sales/process/version-check/

We could do with better documenting the suite of UI components that make up the GitLab Version Check.

I started this in an Issue thread and most could be translated to a .md doc file for future reference

Link to get started: #387719 (closed)

Propsoal

1. Create/update the documentation on GitLab Version Check UI