Fix: groups.py doctest fixes (#4374)

* part of #3925 (doc test mega issue)
* Fixed all failing doctests in groups.py using sphinx directives
   NOTE: These tests will not pass using `pytest -v --disable-pytest-warnings --doctest-modules` 
   since I used sphinx directives to allow for concise example code. This can be changed but the 
   documentation will become more verbose.
* Updated CHANGELOG

package/CHANGELOG

@@ -32,6 +32,7 @@ Fixes
    `DistanceMatrix.run(frames=...)` (PR #4433)
  * Fix doctest errors of lib/picklable_file_io.py (Issue #3925, PR #4371)
  * Fix deploy action to use the correct version of the pypi upload action.
+ * Fix groups.py doctests using sphinx directives (Issue #3925, PR #4374)
 
 Enhancements
  * Added a tqdm progress bar for `MDAnalysis.analysis.pca.PCA.transform()`