Moving user documentation from main homepage to nipype_tutorial

[miykael]

As discussed in #35 with @djarecka and @satra, the goal is to move the user documentation from Nipype's main homepage to nipype_tutorial.

As a first step, I've taken all the documentation from the main page and put them into jupyter notebooks, which can be found here.

@djarecka, how do you want to work on them? Some of those new user_docs can be merged with the ones already existing under Basic Concepts (i.e. MapNode, Function Node, JoinNode, etc.). The rest could perhaps be put under something like "Advanced Concepts". What do you think? (most of them can be merged with the z_notebooks*.ipynb, mentioned in #28)

Also, do you agree, that we can get rid of the following pages:
http://nipype.readthedocs.io/en/latest/users/interface_tutorial.html
http://nipype.readthedocs.io/en/latest/users/tutorial_101.html
http://nipype.readthedocs.io/en/latest/users/tutorial_102.html
http://nipype.readthedocs.io/en/latest/users/tutorial_103.html

[satra]

Also, do you agree, that we can get rid of the following pages:
http://nipype.readthedocs.io/en/latest/users/interface_tutorial.html
http://nipype.readthedocs.io/en/latest/users/tutorial_101.html
http://nipype.readthedocs.io/en/latest/users/tutorial_102.html
http://nipype.readthedocs.io/en/latest/users/tutorial_103.html

yes

[djarecka]

@miykael - that was also my concern, some of the notebooks repeats concepts from the existing tutorial notebooks. If @satra is ok that we won't keep the original content of the docs, merging some of them to the existing ones and putting rest to "Advanced Concept" sounds good to me.

just, please wait with merges to "Basic Concepts" until my PR is finished

[miykael]

@djarecka, no worries. I will wait with any merge until the other PR is finished.