Add attributes to soil and litter example data

[jacobcook1995]

Description

This PR adds attributes to the soil and litter example data files. I have checked the docs and it all seems to render nicely

Fixes #1069

[vgro]

LGTM

[codecov-commenter]

Codecov Report

❌ Patch coverage is 0% with 46 lines in your changes missing coverage. Please review.
✅ Project coverage is 94.60%. Comparing base (a80a7c4) to head (95f6623).
⚠️ Report is 1130 commits behind head on develop.

Files with missing lines	Patch %	Lines
...ample_data/generation_scripts/soil_example_data.py	0.00%	28 Missing ⚠️
...ple_data/generation_scripts/litter_example_data.py	0.00%	18 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop    #1070      +/-   ##
===========================================
- Coverage    95.24%   94.60%   -0.64%     
===========================================
  Files           79       79              
  Lines         6891     6937      +46     
===========================================
  Hits          6563     6563              
- Misses         328      374      +46     

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