Expose page template API for navigation and toc

[jonas]

Currently, $page.navigation$ and $page.toc$ both generate a HTML string with a predefined structure. For themes that require a more custom structure and CSS classes it would be nice to expose a template API to walk the page and page header hierarchy.

Ran into this issue with paradox-material-theme and had to write a lot of JavaScript to get the required structure.

This is mainly useful when porting themes to Paradox.