Update docs & tutorials according to new pip installation method

[BruegelN]

The docs hosted on https://libigl.github.io/libigl-python-bindings/ (generated from tutorial/) still mention installation process via conda and contain quite a lot of references/badges from anaconda.

@alecjacobson somehow I can't assign myself, feel free to assign it to me. I'll try to dig into the mkdocs thing through the course of this week :)

[BruegelN]

I've noticed that both tutorial/tutorials.ipynb and tutorial/tut-chapter7.ipynb aren't on https://libigl.github.io/libigl-python-bindings/. They're not even included in mkdocs.yml. The former is actually no a valid notebook file. Should they be part of the docs or not? I'll leave them aside for now.

[BruegelN]

Hi @alecjacobson,

I'd like to use this issue to keep track of the things that need to be done to update and improve the docs.
Once #187 (just the code) is merge one can merge #192 (gh-pages) to actually update the docs.

After a somewhat recent version of the docs are online I'll proceed with versioned docs. Once I've figured that out I'll create another MR.

[alecjacobson]

I believe this all got merged correctly. Please reopen if I screwed it up.