Update contributor docs

[davidorme]

Description

This PR is to update the contributor/developer docs to make them up to date and fit for an external audience.

Todo:

• [x] Import similar and recently updated docs from pyrealm, which has a very similar contributor setup.
• [x] Add new docs to table of contents and add some basic tags on outdated or new content.
• [x] Fix build errors to allow docs to be review online.
• [x] Convert pyrealm docs for virtual_ecosystem
• [x] Review content of outdated files and delete or port to new files as required.
• [x] Fix up the table of contents to the revised content.

Fixes #498

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
• [x] 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
• [ ] Relevant documentation reviewed and updated

[davidorme]

@vgro I've moved the files in. Now we need to review and adapt.

[codecov-commenter]

Codecov Report

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

Project coverage is 94.91%. Comparing base (63fceb6) to head (a0bceb2).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## develop #499 +/- ## ======================================== Coverage 94.91% 94.91% ======================================== Files 70 70 Lines 3857 3857 ======================================== Hits 3661 3661 Misses 196 196 ```

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[vgro]

I changed all the references from pyrealm to virtual_ecosystem and highlighted a few issues where I wasn't sure if that was still up to date.

[davidorme]

@TaranRallings and @jacobcook1995

Vivienne and I have been over this and updated for VE - can you have a look over the Development section updates. I've built the branch:

https://virtual-ecosystem.readthedocs.io/en/498-update-contributor-documentation/

Ignore the "The design of the Virtual Ecosystem" section. That still needs work.