Add documentation about Wasm extension internals

[mathetake]

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
• 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:

• Add end user documentation for WASM extension. #9462
• Support WebAssembly (WASM) in Envoy #4272
• doc: reorganize the API reference of Wasm extensions #15360

[dio]

cc. @phlax

[mattklein123]

In general WASM docs are at 0% right now. It's a known issue that we need a massive push on docs.