Update documentation for beta1

[polsys]

• Update docs to match API changes
• Add a page for utility code snippets (closes Add useful code snippets #58)
• Add API reference based on the docstrings (manually updated; closes Add API reference to documentation #43)
• Update package branding to focus more on correlation analysis instead of information theory

I will merge this PR a few days before publishing Beta1 in order to review the docs online. There is a warning about out-of-sync docs which should be removed before finishing #41.

[polsys]

Still to be done:

• Review older pages
• Tweak the theme to make the content area slightly wider
• Improve the pictures (transparency, size, etc.)