Documentation: Start troubleshooting guide focused on analyzing Envoy statistics

[sunjayBhatia]

Please describe the problem you have

Users often need to analyze performance or other issues using Envoy statistics, which there are a lot of and Contour sets up in a particular way with stat prefixes, how we set up Listeners, Clusters, etc.

We should start a guide to help users understand the key Envoy statistics to look at and how they might be used

This guide does not have to be complete this first pass, but should be set up so contributors can add to it