BUG: `DataFrameGroupBy.value_counts` includes non-observed categories of non-grouping columns
- [X] closes #46357
- [X] [Tests added and passed](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#writing-tests) if fixing a bug or adding a new feature
- [X] All [code checks passed](https://pandas.pydata.org/pandas-docs/dev/development/contributing_codebase.html#pre-commit).
- [X] Added an entry in the latest `doc/source/whatsnew/vX.X.X.rst` file if fixing a bug or adding a new feature.

This PR has not been reviewed for 1 month. @rhshadrach as you were involved in the issue description, would you like to have a look, or should I wait/ping for the review from a specific maintainer ? @jreback @rhshadrach do you have any other comments to add on this PR or should we consider merging it (after rebase) ? Unsuccessful job seems to be related to the flakyness of https://github.com/pandas-dev/pandas/pull/45651 @jreback @rhshadrach `DataFrameGroupBy.value_counts` was added in pandas-1.4.0

Firstly. do we need the extensive release note. These are normally reserved for bug fixes that have a significant change in behavior that might be considered a breaking change by users.

Also because `DataFrameGroupBy.value_counts` is a new feature we could consider backporting to 1.4.x. 
no backport in this
and i think does need a big note as this is a significant change that's not obvious @rhshadrach I fixed the whatsnew Job failure is due to the unavailability of Github Actions This pull request is stale because it has been open for thirty days with no activity. Please [update](https://pandas.pydata.org/pandas-docs/stable/development/contributing.html#updating-your-pull-request) and respond to this comment if you're still interested in working on this. Apologies @LucasG0 - this fell off my radar. Can you resolve conflicts.

@simonjayhawkins - are you good here? No worries @rhshadrach, conflicts resolved. @simonjayhawkins - friendly ping > @simonjayhawkins - friendly ping

My comments were related to whether this should have been a targeted backport or not.

But I'm not going to block. @jreback - friendly ping thanks @LucasG0 this is very nice! and thanks @rhshadrach for all the work on this as well.