Documentation of new groupings of individuals

[MImmesberger]

• Added new grouping to GEP 1.
• Translated the hh_concepts document to english and edited it slightly to represent the current state of development.

As I presume that this document will eventually be replaced by some tutorial or proper documentation, I didn't put too much effort in making the document appealing for new users. LMK if you think that's necessary though.

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 89.53%. Comparing base (355a113) to head (b17e370). Report is 11 commits behind head on main.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## main #706 +/- ## ======================================= Coverage 89.53% 89.53% ======================================= Files 52 52 Lines 3688 3688 ======================================= Hits 3302 3302 Misses 386 386 ```

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[MImmesberger]

I incorporated most of your comments. For each tax and transfer, we now have two or three Subheadings: "Description", "Aggregation unit" and "Pointers". Aggregation unit is only present if there is aggregation to the unit level. Pointers specify which pointers are used to i) create endogenous groups or ii) establish relationship between individuals (e.g. parent-child links for Kindergeld).

Still have to look at the Wohngeld comment.