[Doc] Add docs for ProtobufNative Schema (#9079)
[Doc] Add docs for ProtobufNative Schema (#9079)

Add docs based on https://github.com/apache/pulsar/pull/8372 and update the green-highlighted parts.
![schema doc architecture](https://user-images.githubusercontent.com/50226895/103272642-5fa07380-49f8-11eb-8098-6eee7f930e93.png)

massakam/pulsar,massakam/pulsar,yahoo/pulsar,merlimat/pulsar,yahoo/pulsar,yahoo/pulsar,merlimat/pulsar,merlimat/pulsar,massakam/pulsar,merlimat/pulsar,merlimat/pulsar,yahoo/pulsar,yahoo/pulsar,merlimat/pulsar,yahoo/pulsar,massakam/pulsar,massakam/pulsar,massakam/pulsar