Improve hierarchical importing

The current hierarchy support assumes proper leveling of pages (in bulleted list) inside Sidebar. Whereas some github wiki pages also contain levels as headings (each bulleted page under a heading, for ex: https://github.com/mixxxdj/mixxx/wiki/New-controller-system). The heading should be a new space (with basic content such as like in parent page) which should act like a table of contents.