Update RPC documentation (#422)

* Update RPC documentation

Updates the outdated documentation for setting up RPC services by
- Removing any notion of the old vscode json-rpc protocol. The neutral term RPC is used instead
- Replacing the outdated loggingServer example with a current version of the taskServer
- Removing sections of the documentation that are now obsolete because the underlying framework code has been reworked

Note: he code snippets are based on eclipse-theia/theia#12581.
So this PR is blocked until  the  referenced PR is approved and merged
(the last section 
Part of eclipse-theia/theia#12368

Author: tortmayr