(WIP) Add navigation systems design pattern

[mcking65]

Resolve #89 and #353 by adding a design pattern section for navigation systems.

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[carmacleod]

Nice work so far, @mcking65 !
This will be a welcome addition to the APG.

I think each of the examples will need a note right above the example (or near the top of the example page) that says something like:

Important: Read Navigation Systems for Sites and Apps before using this example.

[carmacleod]

In case it's helpful, here are the relevant ARIA attributes for each example.
Wasn't sure whether or not to add aria-label[ledby] as well.

[jongund]

@mcking65
@carmacleod
Matt thank you for getting this section started, I think it is important.

A few weeks ago I started on a menu button example for "Mythical University":
menu button navigation example

Just need to add regression tests, the most painful part of developing an example, given the unreliability and instability of the AVA testing system.

I think we also need to talk about the following issues:

1. Number of tab stops in the navigation "menu"
2. Visual affordances for indicating interaction
   2.1 Down arrow before for disclosure menus (e.g. like details/summary)
   2.2. Down arrow after for popup menus (e.g. menubars and menu buttons)
3. The NAVIGATION landmark should be used for navigational links for the website/app, not collection of links to external resources
4. Have a tab only example of the navigation "menu"
5. Have a mega menu example