Add user docs demonstrating demography traits and crown properties

[davidorme]

Description

This PR is mostly about creating some draft intro docs for users for the demography classes and methods. I have also added some __repr__ methods to classes and a few information attributes to classes.

Since this is a doc heavy PR, I've set the branch to build on RTD. The new docs are the ones under this TOC heading.

https://pyrealm.readthedocs.io/en/312-add-user-docs-demonstrating-demography-traits-and-crown-properties/users/demography/module_overview.html

Fixes #312 (issue)

Type of change

• 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

• 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

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 87.50000% with 6 lines in your changes missing coverage. Please review.

Project coverage is 95.70%. Comparing base (1f315ba) to head (7c47deb).
Report is 183 commits behind head on develop.

Files with missing lines	Patch %	Lines
pyrealm/demography/t_model_functions.py	85.00%	3 Missing ⚠️
pyrealm/demography/crown.py	80.00%	2 Missing ⚠️
pyrealm/demography/flora.py	94.44%	1 Missing ⚠️

Additional details and impacted files

@@             Coverage Diff             @@
##           develop     #313      +/-   ##
===========================================
+ Coverage    95.29%   95.70%   +0.41%     
===========================================
  Files           28       34       +6     
  Lines         1720     2469     +749     
===========================================
+ Hits          1639     2363     +724     
- Misses          81      106      +25     

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