Migrate site to Read the Docs

[humitos]

Built docs at https://python-tutorial-es--28.org.readthedocs.build/

Things to consider:

• [x] PDF builder is not rst2pdf in Read the Docs (but it builds properly)
• [x] We don't have the index page anymore (http://docs.python.org.ar/tutorial/)
• [ ] We should kill 2.7 docs
• [ ] Probably Django 1.8 as well
• [x] We need to add a CNAME in the DNS (see https://docs.readthedocs.io/en/stable/custom_domains.html) to use https://tutorial.python.org.ar

Closes https://github.com/PyAr/pyar_infra/issues/47