docs: add examples to DataFrameGroupBy and SeriesGroupBy

[chelsea-lin]

This change is adding example codes to all methods of DataFrameGroupBy and SeriesGroupBy.

• Ensure the tests and linter pass
• Code coverage does not decrease (if any source code was changed)
• Appropriate docs were updated (if necessary)
  • Link to the staging document with this change
  • For example, SeriesGroupBy.cummax: https://screenshot.googleplex.com/zpAnREPUsp9oBUK

Fixes internal bug: 341122015🦕

[ashleyxuu]

You can do a staging following: https://g3doc.corp.google.com/devrel/cloud/infra/devrel_services/g3doc/dev/content_automation/cloud_rad/python.md?cl=head#staging-local-changes-from-github-to-devsite to stage docs manually

Example PR:

• docs: code samples for Series.{add, replace, unique, T, transpose} #287
• docs: update bigquery connection documentation #499

[ashleyxuu]

You can put the staging links in PR the description.