[MNT]: Document and deprecate proj3d

### Summary

`mplot3d.proj3d` does not have docstrings on half the functions, and is not user-facing so all the functions should be deprecated / privatized.

Some discussion in https://github.com/matplotlib/matplotlib/pull/23449. That MR removed all code using `view_transformation`, so that should be marked for eventual removal after deprecating it.

### Proposed fix

_No response_