Add explanatory diagram to Rendering overview article

[avivace]

Re:

**Exception**: an OBJ with an OAM X position of 0 (thus, completely off the left side of the screen) 
always incurs a 11-cycle penalty, regardless of `SCX`.

in the new Rendering Overview article

#501 (comment)

[ISSOtm]

The diagram should give examples of "regular" cases as well; something like https://github.com/gbdev/pandocs/blob/master/src/imgs/Objects_vertical_position.png