Azure: No documentation for onboarding new IDPs with entitlements v2

MR 20 introduced the logic to expect x-user-id header from Istio Envoy Filters. We need documentation on how a customer with a new IDP can onboard with this service

Documentation should explain:

1. How do they create a filter, what are the requirements?
2. Are there additional Istio configuration that they need to set?
3. Any other details so that a customer can go from a new deployment to having onboarded with their new IDP for the environment?