Incorporate menu enhancements from accessibility manual

[yndajas]

Bring in enhancements to the navigation menu design from the accessibility manual:

• Redesign navigation accessibility-manual#90
• Change nav section toggle to button accessibility-manual#93
• Add hamburger menu accessibility-manual#95

Additional considerations:

• the maximum depth of pages in the playbook is higher than in the accessibility manual. I believe the accessibility manual menu is currently set up to handle pages two levels deeper than the root/home page (i.e. immediate children of sections). The playbook goes deeper than this