[Python] Add user guide documentation for Datasets API

[asfimport]

Currently, we only have API docs (https://arrow.apache.org/docs/python/api/dataset.html), but we also need prose docs explaining what the dataset module does with examples.

This can also include guidelines on how to use this instead of the ParquetDataset API (depending on how we end up doing ARROW-8039), this aspect is also covered by ARROW-8047

Reporter: Joris Van den Bossche / @jorisvandenbossche
Assignee: Joris Van den Bossche / @jorisvandenbossche

Related issues:

• [Python][Documentation] Document migration from ParquetDataset to pyarrow.datasets (relates to)

PRs and other links:

• GitHub Pull Request #6779

_{Note: This issue was originally created as ARROW-8063. Please see the migration documentation for further details.}

[asfimport]

Krisztian Szucs / @kszucs:
Issue resolved by pull request 6779
#6779