Warn when no camera exists to render an object

**What problem does this solve or what need does it fill?**

Beginners commonly attempt to get started with adding 2D/3D/UI graphics, forget to add a camera, and then are confused when nothing appears.

**What solution would you like?**

Warn when no camera of the appropriate type exists.

**What alternative(s) have you considered?**

Improve documentation further, especially in the Bevy book.

**Additional context**

The exact timing / details of when this should trigger need some work by others who understand the rendering pipeline more clearly.

We should also be careful not to create unsilenceable warnings in cases where the lack of camera may actually be intended.