* Moved all notebooks to docs/notebooks.
Now all notebooks are in the same place, thus all are subject
to auto-doc generation at readthedocs.io and to automated testing
with travis.
Some notebooks are too slow, exclude them at docs/conf.py:exclude_patterns.
Cleanup a bit the section headings in notebooks so that they show
up well in readtehdocs.io.
* Increase the cell timeout for executing notebooks
* Exclude also the neural network notebook from auto-generation (timing out)
* Disable the score_matching notebook from auto-doc (travis does not have sklearn)
Testing is done by running "jupyter nbconvert --to notebook" and
then parsing the resulting notebook to look for errors.
One can declare expected errors, and the test will fail if those
are missing.
In the process of doig this, found and fixed a bug in the autodiff_cookbook
notebook.
* Cleaned up use of section levels
* Renamed ma to multiply_add and sq_add to square_add
* Other minor clarifications
* Separated the Colabs into Tutorials and Advanced Tutorials
