Add Azure Blob Storage configuration documentation
This documents the changes needed to store objects in Azure:
- Rails changes: !38882 (merged)
- Rails changes for custom domains: !40694 (merged)
- Workhorse changes: gitlab-workhorse!555 (merged)
- Workhorse changes for custom domains: gitlab-workhorse!575 (closed)
Closes #25877 (closed)
Edited by Stan Hu