reference links added

[vgro]

There were a few reference links missing in the VE theory which I now added.

Fixes #504

Type of change

• [ ] New feature (non-breaking change which adds functionality)
• [ ] Optimization (back-end change that speeds up the code)
• [x] 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 (c406599) to head (cdbe4bc).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## develop #505 +/- ## ======================================== Coverage 94.91% 94.91% ======================================== Files 70 70 Lines 3857 3857 ======================================== Hits 3661 3661 Misses 196 196 ```

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