Add documentation about Wasm extension internals

Given that Wasm extension mechanism seems more complex than standard built-in extensions, it is useful provide the dedicated documentation about Wasm extension internals for end-users and Envoy core developers.

This would include, for example,
- How Envoy is integrated with [Proxy-Wasm C++ Host](https://github.com/proxy-wasm/proxy-wasm-cpp-host)
- How runtimes interact with Envoy
- The Threading model
- How VMs are initialized and Wasm binaries are loaded
- Envoy specific implementation details
- How to provide Envoy specific ABI
- Considerations and limitations (e.g. memory consumptions, etc.)
- How to develop statically linked NullVm Wasm plugins

and could be based on https://github.com/proxy-wasm/spec/blob/master/docs/WebAssembly-in-Envoy.md

Related issues:
- https://github.com/envoyproxy/envoy/issues/9462
- https://github.com/envoyproxy/envoy/issues/4272
- https://github.com/envoyproxy/envoy/pull/15360