Migrate documentation to source code

[EpicEric]

Maintaining a documentation separate from the source code is cumbersome. The following steps should be taken to host the docs in a simpler way:

• Move all documentation from the docs folder to the source code.
• Generate Markdown from source code.
• Use https://github.com/ponylang/mkdocs-theme to provide the official Material theme.
• Migrate from GitBook to Mkdocs.
• Set up Travis to auto-host new master versions of the documentation on Mkdocs.

[EpicEric]

I've set up a solution with CircleCI instead of Travis.