docs: Add initial stab at integration guide
docs: Add initial stab at integration guide

This is not complete (e.g., more detail on management APIs would be
great, detail on CLI invocation would be nice, detail on partial
evaluation would be very helpful, etc.) but it provides a starting
point.

Fixes #372

Signed-off-by: Torin Sandall <933af5bec0bc58056526d55ca6c7b45fcf14cc48@gmail.com>

open-policy-agent/opa,open-policy-agent/opa,tsandall/opa,tsandall/opa,open-policy-agent/opa,tsandall/opa,tsandall/opa,open-policy-agent/opa,tsandall/opa,open-policy-agent/opa,tsandall/opa,open-policy-agent/opa