Add a description to "Project an atom onto the canvas" example

[NataliaDSmirnova]

Is your feature request related to a problem? Please describe.
The examples show two ways of projection an element to the canvas. But it's not clear from the page why the red circle is visible, while the green circle is hidden when the element is out of the viewport (see screenshot). Now it may confuse a user and make them think that something is wrong.

Describe the solution you'd like
I would suggest to add a description label to the page. Something like "The page demonstrates two ways of an element projection to the canvas...."

Additional context
See https://miew.app/examples/projected.html for details.