Pages: Upgade guide to QUnit 2.x

Fixes #86 

First draft for an upgrade guide. There are no links to this page, yet. There are layout issues, may need to port some CSS from jqueryui.com.

Would be good to get some feedback on the format. Also, am I missing something?
Should we explain the reason? 

`This way, the assertion will be directly related to its test context, without leaking to other tests even on async cases.`