Docs to clarify what various components are used for

[ptodev]

I'd like the new docs to be the place we refer people when someone asks whether the Agent can do something, or whether it can be used in a certain situation with a certain telemetry. We currently don't have a doc which lists all of this info, and we need to rephrase it manually in conversations.

This PR is a draft because I am not sure of the best location for this, and I'm not sure how it is best represented.

[ptodev]

I think the ToC looks nice with the proposed titles:

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