Call the template instance `template`, not `this`

But keep some occurrences of `this` for clarity; for example, “in a rendered callback you can use `this.findAll`”.
Deprecate UI.insert and isAttached in docs

Also:
* Rework UI.render and UI.remove docs
* Get rid of reference to UI.destroy
* Document optional parentView argument to UI.render[withData]
Don’t hype UI.Template
More docs fixes from MDG feedback
findView becomes getView, has no viewName arg

Also, make UI.templateInstance() not throw, to match the docs.