Incorporate menu enhancements from accessibility manual

[yndajas]

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

• https://github.com/dxw/accessibility-manual/pull/90
• https://github.com/dxw/accessibility-manual/pull/93
• https://github.com/dxw/accessibility-manual/pull/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