GitLab Version Check - Document Fetching/Caching

What / Why

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

We could do with better documenting the process of how GitLab fetches and caches the version data from https://gitlab.com/gitlab-services/version-gitlab-com

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

Link to get started: #352355 (comment 1236046101)

Propsoal

1. Create/update the documentation on GitLab Version Check fetching and caching