Document DataSources (and DataSinks of course)

[schuderer]

Although the documentation is getting more and more complete, we still lack more explicit documentation of the existing datasources/datasinks:

• Name of the DataSource/DataSink (document those who belong together (e.g. FileDataSource/FileDataSink directly under each other if possible):
• Is it included? -- or available externally (e.g. in examples), in which case specifying them with "plugins" is necessary.
• What it is for, what it does
• How to configure
• How to use (including code example(s)).
• Special features and considerations.

Ideally document all included datasources/sinks in a specific file docs/datasources.rst

Right now, some of their documentation is buried in the API documentation of the mllaunchpad.resource. Maybe there's a way to lift (part of) this API documentation from there into docs/datasources.rst