add more content to Documentation page

Fixes #15
move to second-level headings for consistency

This was done to the rest of the site in ea650b9b.
update config to fix pygments deprecation warning

When running the `jekyll serve` command, there is a warning:

    Deprecation: The 'pygments' configuration option has been renamed to
    'highlighter'. Please update your config file accordingly. The allowed
    values are 'rouge', 'pygments' or null.