docs: verify config snippets are valid

This is an aspirational work item, but it would be really nice if we could verify all of the config snippets in the docs are actually valid config. The way I would imagine this working is:
1) Develop a new Sphinx plugin for an Envoy config snippet. This would identify the associated proto that it should load into.
2) During doc generation, go through all snippets, and try to load the config YAML into the actual proto.

This would catch stale config. This might be a good GSoC project or a good item for someone that wants to do something really useful that is not C++.