Doc make.py clean
@jorisvandenbossche @datapythonista currently `_sphinx_build` in `doc/make.py` always calls `clean`. Do you know why that was added?

My doc-build process in https://github.com/pandas-dev/pandas-release assumes that I can do `make.py html` and `make.py latex_forced`.

An option to not clean would be fine by me. 