Clarify use of service/robot accounts

[llarsson]

Describe the improvement

This page does not explain how automation efforts with service/robot accounts "authenticate" and carry out their work. It all looks like it's always Dex, which it isn't.

I would love for us to have:

• a page that collects the information regarding how to work with service/robot accounts in all our endpoints;
• an figure about authentication that shows that and a link to said page; and
• that everywhere where we now discuss service or robot accounts simply pull in information snippets from the page, so that we have a smooth way to avoid repeating ourselves.

Additional context

LinkedIn question with context that Dex currently doesn't support machine-machine automation via the client credentials flow, which makes it a very relevant question to ask.