Cleaner docs with snippets for using Minibatcher

## Issue

Following on from #537. We should provide more information to users about the procedure users should use in setting up the minibatching experiment. In particular we are missing
- That once we have created the Observation series, the user does not need to manually call `update_minibatcher` - just update ensemble.
- To retrieve the next minibatch indices one should call e.g. `get_current_minibatch(get_observation_series(ekp))`

## Possible solution
- Add the API `get_current_minibatch(ekp)` so that people don't need to retrieve the observation series,
- Add code snippets to the docs (front and centre) on how to call ekp with minibatching, including `update_ensemble!` and `get_current_minibatch(ekp)`

Start by reading the existing minibatching documentation and the APIs mentioned in the issue: update_minibatcher, update_ensemble!, get_observation_series, and get_current_minibatch. Add front-and-centre snippets showing ensemble updates and retrieval of the next minibatch, and determine whether the proposed get_current_minibatch(ekp) API is needed; the documentation should clearly show the complete setup procedure.

Written by the indexing model from the issue text.