Add missing API docs for bh.Histogram

[Saransh-cpp]

For some reason, the API doc of Histogram is missing from the documentation website. Histogram is in a private module, but it is exported in __init__.py. Furthermore, the module boost_histogram itself has no docstrings to display.

This was briefly mentioned in scikit-hep/hist#445.

[henryiii]

Does this get removed if you rerun nox -s build_api_docs?

[Saransh-cpp]

Oh yes, it reverts the changes. Looking into it.

[Saransh-cpp]

Adding --private brings in every private module to the docs. We could only include hist.py by excluding other private files, but I can't see a way to exclude private members of hist.py after using --private.

The templates could be changed but the documentation for it is almost non-existent. The only available examples are in the Sphinx project, which I think were taken as it is for boost-histogram.

[henryiii]

It's modified slightly, but yes, it was.

We could hack it by reading the file and then writing it after the generation step. It would still work in nox but not by hand, which IMO is fine for now.

Ideally, I'd like to get rid of the private files and instead move everything to where it is supposed to be, then use __all__ and module level __dir__ to hide the private parts. That wasn't supported everywhere when boost-histogram was written (in fact, 3.7 was probably pretty new). But that's for later.

[henryiii]

Thanks!