Better documentation for programming Envoy extensions

As it stands, the documentation for extending Envoy is [pretty thin](https://www.envoyproxy.io/docs/envoy/latest/extending/extending). The [envoy-filter-example](https://github.com/envoyproxy/envoy-filter-example) project does provide a nice example but it'd be good to have a more step-by-step, narrative-style tutorial surrounding this.

I will be taking on this work. If anyone has recommendations for resources, possible directions, sub-topcs, etc., please feel free to weigh in here.