improve jsonrpc js docs

[Simon-Laux]

• change export name from DeltaChat to WebsocketDeltaChat
• export Event and ContextEvents type to include them in the documentation.
• dedicated readme for @deltachat/jsonrpc-client. so there is something more useful displayed on npm and on https://js.jsonrpc.delta.chat
• update link to echo bot in deltachat-rpc-server
• update deltachat-jsonrpc readme
• run prettier formatter on some readme files
• add instructions on how to use on an unsupported platform

[Simon-Laux]

needs rebase