[Datasets] Last-mile preprocessing docs. (ray-project#20712)

Datasets docs for last-mile preprocessing, particularly geared towards ML ingest. This gives groupby, aggregations, and random shuffling examples in the overview page (not present previously), adds some concreteness to our last-mile preprocessing positioning, and provides some preprocessing recipes for a few common transformations.

Revert change to set debug info to -g1 (ray-project#19900)

Fix windows build environment breakage (ray-project#19019)

Fix windows build environment breakage (ray-project#19019)

[dashboard] Don't start dashboard agent when missing dependencies (ra…

…y-project#17966)

LINT changes.

fall back to manual pin version

fall back to manual pin version

change to ray version 1.5.0

Rerun due to py36 wheel build failure