Add warning for v-app-bar placement in v-app

[bartduisters]

Added a warning about the placement of the v-app-bar component.

[J-Sek]

Not sure if it helps when it is so vague. What was the problem? Missing v-app or placing v-app-bar deeper in the DOM tree? It should also work with v-layout, but going with too much detail can confuse readers.

[bartduisters]

A student was trying out components, so the v-app-bar was copy pasted as shown in the example. There then was a warning/error in the console about layout. I then took a look and assumed v-app-bar should be placed within v-app, which fixed it.

This was my attempt to make the documentation page self containing. Someone new to Vuetify doesn't know it should live within a v-app or v-layout, is there a specific page/piece of information I missed?

[KaelWD]

The app components' pages should probably all link to https://vuetifyjs.com/en/features/application-layout/ somehow. (Also that page barely even mentions v-app and v-layout or the difference between them)