Add a contents pane option to the MooseDocs navigation extension

[joshuahansel]

Motivation

It can be difficult to navigate some longer pages in MooseDocs and be able to see the overall page organization with the different heading levels, leading to some confusion.

Design

This would add a toggle to MooseDoc's NavigationExtension called contentspane or something (false by default), and then a user could enable it on a page like

!config navigation contentspane=True

(some page content)

This would produce a left pane with hyperlinks that looks like:

Food
  Fruits
    Apple
    Banana
  Vegetables
    Carrots

Impact

Improved readability of documentation pages.