Use docs extra for building docs instructions

With the `docs` install extra in setup.py we can simplify the
setup for building the docs by avoiding the now redundant
docs/doc-requirements.txt and docs/environment.yml.
One change is needed in setup.py though which is to add the
`sphinx-rtd-theme` package to the setup.py `docs` extra install
package list which was in doc-requirements.txt and environment.yml
but missing from setup.py.

CONTRIBUTING.rst

@@ -174,17 +174,17 @@ To build the documentation you'll need `Sphinx <http://www.sphinx-doc.org/>`_,
 To install (and activate) a `conda environment`_ named ``notebook_docs``
 containing all the necessary packages (except pandoc), use::
 
-    conda env create -f docs/environment.yml
+    conda create -n notebook_docs pip
     conda activate notebook_docs  # Linux and OS X
     activate notebook_docs        # Windows
-    pip install .
+    pip install .[docs]
 
 .. _conda environment:
     https://conda.io/docs/user-guide/tasks/manage-environments.html#creating-an-environment-from-an-environment-yml-file
 
 If you want to install the necessary packages with ``pip``, use the following instead::
 
-    pip install . -r docs/doc-requirements.txt
+    pip install .[docs]
 
 Once you have installed the required packages, you can build the docs with::
 

setup.py

@@ -118,7 +118,8 @@ for more information.
     extras_require = {
         'test': ['nose', 'coverage', 'requests', 'nose_warnings_filters',
                  'nbval', 'nose-exclude', 'selenium', 'pytest', 'pytest-cov'],
-        'docs': ['sphinx', 'nbsphinx', 'sphinxcontrib_github_alt'],
+        'docs': ['sphinx', 'nbsphinx', 'sphinxcontrib_github_alt',
+                 'sphinx-rtd-theme'],
         'test:sys_platform != "win32"': ['requests-unixsocket'],
     },
     python_requires = '>=3.5',