Add missing docstring examples

[Saransh-cpp]

All the # TODO: Add an example here - see https://github.com/scikit-hep/vector/issues/194 lines in https://github.com/scikit-hep/vector/blob/main/src/vector/backends/numpy.py

[pandyah5]

I can add a few of these docstrings. Will link a PR to the issue when it is ready!

[Saransh-cpp]

Thanks! Let me know if any help is required!

[pandyah5]

Hi @Saransh-cpp work has been keeping me busy lately hence the delay. The PR is now up for review, I have tested the demo commands myself and verified them. If there are any changes I should make, feel free to leave a comment on my PR. Have a great weekend :)