[Python][Documentation] Document migration from ParquetDataset to pyarrow.datasets

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We need documentation describing a migration path from ParquetDataset, at least for the basic user facing API of ParquetDataset (As I read it, that's: construction, projection, filtering, threading, for a first pass). Following this we could mark ParquetDataset as deprecated, building features needed by power users like dask and adding those to the migration document

Reporter: Ben Kietzman / @bkietz

Related issues:

• [Python] Long-term fate of pyarrow.parquet.ParquetDataset (is related to)
• [Python] Add user guide documentation for Datasets API (is related to)

_{Note: This issue was originally created as ARROW-8047. Please see the migration documentation for further details.}

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Joris Van den Bossche / @jorisvandenbossche:
I also created ARROW-8063 for general user guide docs for the dataset module

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Krisztian Szucs / @kszucs:
You added examples in the guide, so I postpone it to the next release.

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Alessandro Molina / @amol-:
Postponing this to version 10

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Apache Arrow JIRA Bot:
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