Draft forms and input validation design pattern section

[mcking65]

The APG 1.0 included a guidance section on forms, but this content could be more effectively presented as a design pattern demonstrating how aria-invalid, aria-errormessage, and aria-describedby can be used.

When working on this issue, there is some old content that may be useful in the file aria-practices-DeletedSectionsArchive.html. The relevant section can be seen here: https://rawgit.com/w3c/aria-practices/master/aria-practices-DeletedSectionsArchive.html#ariaform