Improve Kafka integration docs

[gvdongen]

The Java-Kafka docs need improvement: https://docs.restate.dev/services/sdk/kafka/#:~:text=RestateContext%20ctx%2C-,Event,-event)%20%7B

It is not really clear when I can/should use:
dev.restate.Event
And when I can/should use:

message MyEvent {
    string id = 1 [(dev.restate.ext.field) = KEY];

    bytes payload = 2 [(dev.restate.ext.field) = EVENT_PAYLOAD];

    // This is optional
    map<string, string> metadata = 3 [(dev.restate.ext.field) = EVENT_METADATA];
}

Improve the docs on parallelism / key requirements / etc.

[gvdongen]

Fixes by #352 #351