Specifies sphinxcontrib-bibtex version for the docs (#270)

This MR specifies the sphinxcontrib-bibtex==2.5.0 to prevent hanging upon
building docs with python3.10. When the version is greater than this, it causes
pip to hang on requirements.txt installation such that building docs is
impossible.

## Type of change

- Bug fix (non-breaking change which fixes an issue)

## Checklist

- [x] I have run the [`pre-commit` checks](https://pre-commit.com/) with
`./orbit.sh --format`
- [ ] I have made corresponding changes to the documentation
- [x] My changes generate no new warnings
- [ ] I have added tests that prove my fix is effective or that my
feature works
- [ ] I have updated the changelog and the corresponding version in the
extension's `config/extension.toml` file
- [x] I have added my name to the `CONTRIBUTORS.md` or my name already
exists there

docs/requirements.txt

 # for building the docs
 sphinx-book-theme==1.0.1
 myst-parser
-sphinxcontrib-bibtex
+sphinxcontrib-bibtex==2.5.0
 autodocsumm
 sphinx-copybutton
 sphinx_design