Documentation for BC's in v2.0

[jhdark]

Proposed changes

Describe the big picture of your changes here to communicate to the maintainers why we should accept this pull request. If it fixes a bug or resolves a feature request, be sure to link to that issue.

Types of changes

What types of changes does your code introduce to FESTIM?

• [ ] Bugfix (non-breaking change which fixes an issue)
• [ ] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to not work as expected)
• [ ] Code refactoring
• [ ] Documentation Update (if none of the other choices apply)
• [ ] New tests

Checklist

• [ ] Black formatted
• [ ] Unit tests pass locally with my changes
• [ ] I have added tests that prove my fix is effective or that my feature works
• [ ] I have added necessary documentation (if appropriate)

Further comments

If this is a relatively large or complex change, kick off the discussion by explaining why you chose the solution you did and what alternatives you considered, etc...

[codecov[bot]]

Codecov Report

:white_check_mark: All modified and coverable lines are covered by tests. :white_check_mark: Project coverage is 95.50%. Comparing base (a02ed4b) to head (7aa0ca6). :warning: Report is 27 commits behind head on fenicsx.

Additional details and impacted files

@@             Coverage Diff             @@
##           fenicsx     #974      +/-   ##
===========================================
- Coverage    95.56%   95.50%   -0.07%     
===========================================
  Files           45       45              
  Lines         3089     3089              
===========================================
- Hits          2952     2950       -2     
- Misses         137      139       +2     

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