Write documentation for SQL inputs

[sebastian-peter]

SQL inputs for timeseries have currently been introduced. The documentation currently only includes details on influx and csv inputs.

At the same time, some sections can probably be moved to general places, since for example csv and SQL input schemas are quite similar.

Furthermore, an overview of standard input units could be provided.

[sebastian-peter]

I think it's important here to include CREATE TABLE statements for all available input and output tables, including recommended index creation etc.