Document OpAMP

[mhausenblas]

Desired feature or idea.
As an operator, I'd like to understand what OpAMP is and how to use it. Currently, there is no docs entry.

Additional context
In addition to explaining concepts such as agent, supervisor, and server, I think it would be great to show them in action based on the OpAMP Go example.

This is related to #1701 and @tigrannajaryan offered to review.

[mhausenblas]

I volunteer to write that docs entry, feel free to assign it to me @svrnm

[svrnm]

That would be awesome @mhausenblas, thanks! We stopped assigning issues, so please just go ahead! :-)

FYI, we had this blog post a while back by @nemoshlag on opamp: https://opentelemetry.io/blog/2022/opamp/, so maybe some of that can be helpful.

[mhausenblas]

https://opentelemetry.io/docs/collector/management/ is live now, closing.