docs: Add architecture document for Management Domains

[jimmarino]

What this PR changes/adds

Adds an architecture document for Management Domains.

Why it does that

Briefly state why the change was necessary.

Further notes

One or more DRs will be added later.

Linked Issue(s)

Related to #4262

[mspiekermann]

While I have nothing against the content (quite the opposite), I would move section 3 out of this specific document. I think it would be better to split the explanation of the management-domain concept and how it is implemented (which is better placed in a story, issues and/or a decision record).

[jimmarino]

While I have nothing against the content (quite the opposite), I would move section 3 out of this specific document. I think it would be better to split the explanation of the management-domain concept and how it is implemented (which is better placed in a story, issues and/or a decision record).

Yes, more implementation specifics will be added in subsequent DRs. This was just intended as an outline of what needs to be done. I think I mentioned that in the associated issue.