Add proper explanation of soil carbon and macronutrient cycles to the soil model documentation

[jacobcook1995]

Description

This PR is an attempt to improve the documentation of the soil carbon and macronutrient cycles. It is currently in very early draft stage, it is mainly being put up as a pull request so that me and @hrlai can both contribute to it.

The documentation for this branch is built here, the relevant section is The Virtual Ecosystem -> Theory -> Soil theory.

Fixes #662

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 94.72%. Comparing base (52f15b8) to head (a8eba94).
Report is 8 commits behind head on develop.

Additional details and impacted files

@@           Coverage Diff            @@
##           develop     #752   +/-   ##
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  Coverage    94.72%   94.72%           
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  Files           75       75           
  Lines         5197     5197           
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  Hits          4923     4923           
  Misses         274      274           

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[jacobcook1995]

@hrlai, I'm starting by working on the soil carbon cycle documentation. I don't think that section is yet in a state where it's worth your time looking at it. I'll let you know once it's reached a good enough state to be worth looking at!