Use MkDocs for the documentation

[rlouf]

This PR switched the documentation from Sphinx to Mkdocs, which comes with better visual defaults and plugins. Markdown has a broader appeal than restructured text for potential contributors.

Landing page:

TODO

• Add API documentation.
• Update the documentation with the current interface
• Add documentation for regex-guided generation
• Add documentation for JSON-guided generation
• Add at least one example

[rlouf]

This is already a big change, will open issues to add documentation and examples.