[MRG] Documentation: added sphinx support and some first docstrings

[NicolasHug]

This PR adds a doc directory, setups sphinx (separate build and source directories) with sphinx-quickstart and adds some first docstring proposals for the estimators.

Should we set up something on ReadTheDocs or equivalent?

I set up the RTD theme because I like it but I'm open to anything of course.

[codecov]

Codecov Report

Merging #54 into master will not change coverage.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master      #54   +/-   ##
=======================================
  Coverage   95.39%   95.39%           
=======================================
  Files           9        9           
  Lines         891      891           
=======================================
  Hits          850      850           
  Misses         41       41

Impacted Files	Coverage Δ
pygbm/splitting.py	98.95% <ø> (ø)	⬆️
pygbm/gradient_boosting.py	88.06% <ø> (ø)	⬆️
pygbm/binning.py	92.98% <100%> (ø)	⬆️
pygbm/grower.py	92.81% <100%> (ø)	⬆️

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[ogrisel]

I am fine with readthedoc.

[ogrisel]

+1 for merging quickly and improving the doc and docstring in later PR once it's easy to read the doc online.

[NicolasHug]

Ok I'll add some docstrings for the grower as well, and merge.

Do we want to spin up a Circle CI that build the docs for each PR like in scikit-learn? Unfortunately RTD does not support it at the moment readthedocs/readthedocs.org#1340

[ogrisel]

I think it's fine just to build a "dev" version of the doc from the master and a "stable" version of the doc from the stable release when we make one.

[NicolasHug]

Ok.

Then I'm not sure what you mean by once it's easy to read the doc online

[NicolasHug]

Merged @ogrisel , the docs are here: https://pygbm.readthedocs.io/en/latest/
and the repo RTD project here: https://readthedocs.org/projects/pygbm/

I flattened everything in index.rst but that's just to see what it looks like.

As I don't have admin rights on the github repo, RTD couldn't set up the hooks so new commits won't trigger the doc buildings. I added you as admin on RTD maybe that will fix it.