507 add more theory to the theory

[vgro]

Based on Rob's feedback that the 'theory is not really theory' but more about the model and implementation, I tried to update the overview and abiotic documentation to add more general principles and set links to implementation pages. Please feel free to comment/rewrite where you feel it is to technical.

you can look at the docs here https://virtual-ecosystem.readthedocs.io/en/507-add-more-theory-to-the-theory/

I also reorganised the file structure.

Fixes #507

Type of change

• [x] New feature (non-breaking change which adds functionality)
• [ ] Optimization (back-end change that speeds up the code)
• [ ] Bug fix (non-breaking change which fixes an issue)

Key checklist

• [x] Make sure you've run the pre-commit checks: $ pre-commit run -a
• [ ] All tests pass: $ poetry run pytest

Further checks

• [ ] Code is commented, particularly in hard-to-understand areas
• [ ] Tests added that prove fix is effective or that feature works
• [x] Relevant documentation reviewed and updated

[codecov-commenter]

Codecov Report

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

Project coverage is 94.91%. Comparing base (b077a59) to head (88aa214).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## develop #508 +/- ## ======================================== Coverage 94.91% 94.91% ======================================== Files 70 70 Lines 3857 3857 ======================================== Hits 3661 3661 Misses 196 196 ```

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Have feedback on the report? Share it here.