Migration: Nginx and Ingress conversion document

In order to achieve a smooth migration from Nginx to Ingress, we need to summarize the list of features/directives in nginx and how to convert them to Ingress model.

It would be better to have following items in the document:

1. Nginx directive name (linked to its document)
2. Priority (High, Medium, Low): The priority of implement the support to this directive
3. Corresponding Ingress/Envoy configuration (linked to its document)
4. How to convert