[DOC] Add documentation changes for time series tasks.

[ravinkohli]

• I'm submitting a ...
  • feature request

Issue Description

Support for time series tasks as introduced in #434, does not include the relevant changes to the documentation. I think before making a new release with time-series tasks, we should also update the documentation for the library. Particularly,

1. in the README.md here, we should add time-series tasks, here we should update whether baselines are used for time series tasks or only for tabular tasks. We should also add a reproducible example for time-series tasks here.
2. In the API we should add time series forecasting API as well as the time series forecasting pipeline.
3. In the manual, we should add information about data preprocessing and the steps in the time-series pipeline. This can be done either by simply appending the text or by making separate pages for both. The same should be done in dev.

Possible Solution

This issue can be resolved either by a single PR or by 2 PRs split between adding time-series forecasting and making the necessary changes for the rest of the documentation.