Resolve "Write documentation for GraphGroup"

This MR updates the documentation such that it adds a Data Group section on the index page and a GraphGroup entry.

Open for discussion / please double-check

• Is the example code useful? If yes, should we invest time to create a test_doc_examples.py and literal includes such that we can be sure that the example codes are also working correctly? However, this seems in this example to be quite a bit of work to create a graph group with multiple subgroups and containers to just show that the basic functionality works. Therefore, I could also think of either leaving out the example codes or extent the docstrings of the functions. What are the opinions on this one?
• Is the documentation understandable and useful?
• Is something missing?

Can this MR be accepted?

• Documentation extended or updated
  • Added the file doc/groups/graph.rst and wrote the GraphGroup documentation entry.
• No code changes (except tested includes)

• Ready for merging
  • Pipeline passes without warnings
  • History cleaned-up or squash option set
  • Changelog entry added
  • Version number bumped to 0.12.4
  • Reviewed & approved by @jeremiastraub
  • Reviewed & approved by @blsqr @herdeanu

Related issues

Closes #136 (closed)