Document supported input formats

[suvayu]

It would be really helpful to have an overview of all supported table formats and the required arguments - we tried to do this here (but only for yearly data, didn't get to write tutorials for our continuous-time (datetime) features).

Originally posted by @danielhuppmann in #8 (comment)

• different kinds of time series
• long vs wide form tables
• common data analysis frameworks (pandas & xarray)
• (WIP) SQLite