Copied docs from .org repo

[erez-o]

Copied relevant docs from https://github.com/yaml/pyyaml.org. I think it's helpful for users if all relevant docs appear inside the repo.

I've also created a docs for PyYaml on https://doxiz.com/PyYAML/. It's a free documentation generator for open source that supports both tutorial pages and apis for Python and C++, customizable using a simple yaml config.

If you're interested, I can transfer ownership and you can customize it to look better than the pyyaml.org website.

Disclosure - I'm the creator of doxiz.com