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 94.79%. Comparing base (abd205d) to head (667e029).

Additional details and impacted files

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##           develop     #752   +/-   ##
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  Coverage    94.79%   94.79%           
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  Files           75       75           
  Lines         5592     5592           
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  Hits          5301     5301           
  Misses         291      291           

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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!

[jacobcook1995]

@hrlai I've finished the soil carbon section of the docs (I still need to work on the soil nutrients section). Let me know what you think, and feel free to make edits if you think there's something incorrect, missing, etc

[hrlai]

@jacobcook1995 I read the new documents and added only minor edits. They look very good as a first pass to me, and I have a much better understanding of the soil module. I think this PR is ready to be merged. There might be some missing items, or things that can added; but they can wait for the next round of revision?

[jacobcook1995]

@hrlai , thanks for the changes, it's really useful to have a second set of eyes on this! I still need to flesh out the macronutrients section (and do the rearrangement that you suggested). I'll do that on this PR and get you to take another look, if that's okay?

[jacobcook1995]

@hrlai I've fleshed out the remaining content and restructured the environment factors section as you suggested. Let me know what you think!

[hrlai]

@jacobcook1995 this looks ready for merging, I have only added a few very minor tweaks and little comments.