Capabilities page for access logging

[kenjenkins]

Pomerium has access logs (from the proxy service), with general request/response information, as well as authorize logs (from the authorize service), which contain some similar information together with deny/allow reasons.

I think it would be useful to explain these concepts in general terms. We have reference pages (Access Log Fields and Authorize Log Fields) that cover how to configure what specific fields should be included in each, but that's a little different than describing what they are and when you might search for one or the other.

We might want to expand the existing Audit Logs page to cover both of these.