Add Gitaly to the architecture of the application

What does this MR do?

Adds Gitaly to the architecture design document of the application

Are there points in the code the reviewer needs to double check?

That the explanation is in line with what the project is and that it's easy to understand.

Why was this MR needed?

Because we are adding a new component to the architecture

Screenshots (if relevant)

N/A

Does this MR meet the acceptance criteria?

• Changelog entry added
• Documentation created/updated
• API support added
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together