get rid of square citation brackets and reference labels

This is ugly: ```evaluated and then refined [Voorendt, 2017].```
So is the reference label:
```
[DK22]
Armen Der Kiureghian. Structural and System Reliability. Cambridge University Press, 2022.
```

[This solution](https://github.com/executablebooks/jupyter-book/issues/1090) looks a bit complicated, but seems to do exactly what's needed.