Draft Math design pattern section

[mcking65]

In the ARIA 1.0 APG, there was a gidance section for math. This content will fit more appropriately in the design pattern section as an example.

The 1.0 guidance content can be found in the file aria-practices-DeletedSectionsArchive.html. The relevant content can be seen here: https://rawgit.com/w3c/aria-practices/master/aria-practices-DeletedSectionsArchive.html#math