Update add-vars.md

[timothy-nunn]

Updated the guide for adding variables, inputs, scan vars, figures of merit, and iteration vars. They needed a refresh due to changes in the Python conversion, recent data structure changes, and enumification.

I have also reduced the need for users to manually increment some variables (was not possible for the scan variables due to circular imports).

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 48.78%. Comparing base (89e1ebe) to head (a1eca61).
⚠️ Report is 9 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4332      +/-   ##
==========================================
- Coverage   48.79%   48.78%   -0.02%     
==========================================
  Files         151      151              
  Lines       29383    29392       +9     
==========================================
+ Hits        14337    14338       +1     
- Misses      15046    15054       +8     

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[timothy-nunn]

Requesting a couple of reviews to ensure these docs make sense and are sufficiently detailed

[clmould]

All minor things, mostly suggested changes, but otherwise happy with the content