PM-525 ReadTheDocs documentation

[germartinez]

Integrates the documentation with ReadTheDocs

• Uses sphinx-js to build the documentation.
• The documentation is generated by ReadTheDocs.
• The old documentation folder (tutorials) has been deleted and the .md and .rst files are now in docs/source

[cag]

Sweet, thanks German!

I'll address some little things when I rebase another PR I have which does some docs stuff. For reference, docs can now be found here: https://gnosisjs.readthedocs.io/en/latest/api-reference.html