doxygen: dropdown for different versions

[gknopf-aranya]

We want to have an easy way to navigate between different versions of the Doxygen docs similarly to how Rust docs sites work.
Notice how there is a dropdown on docs.rs that allows the reader to select different versions of the docs:
https://docs.rs/aranya-client/0.4.0/aranya_client/

Read these first for some implementation ideas:
https://oleksandrkvl.github.io/2024/06/05/multi-version-doxygen.html
https://github.com/root-project/doxyvers