Adding back "Datastore" section to docs. - #994

Adding back "Datastore" section to docs.

* feat: add `AvroOptions` to configure AVRO external data

Also:

* Unify `ExternalConfig` class to use `_properties` for everything. This does
  result in more code, but it should make maintenance easier as it aligns with
  our other mutable resource classes.
* Adds `bigtable_options`, `csv_options`, and `google_sheets_options`
  properties. This aligns with `parquet_options`.

* remove unnecessary check for options in to_api_repr

* add missing tests for to_api_repr

* remove redundant type identifiers

* docs: streamline acl/iam docs

* update docs and guides

* 🦉 Updates from OwlBot post-processor

See https://github.com/googleapis/repo-automation-bots/blob/main/packages/owl-bot/README.md

* exclude README from owlbot

* revert owlbot readme reverts

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Co-authored-by: Dan Lee <71398022+dandhlee@users.noreply.github.com>
Co-authored-by: Owl Bot <gcf-owl-bot[bot]@users.noreply.github.com>