Doc: update ShapeList, ShapeElementList, and other doc with clearer mentions of pyglet.shapes #2608

What needs improvement

The ShapeElement* doc mentions that these lists aren't updatable, but they don't include cross-reference:

A ShapeElementList is a list of shapes that can be drawn together in a back for better performance. ShapeElementLists are suited for drawing a large number of shapes that are static. If you need to move a lot of shapes it’s better to use pyglet’s shape system.

Proposed fix

Add cross-references to:

1. https://pyglet.readthedocs.io/en/latest/programming_guide/shapes.html
2. specific shapes in the https://pyglet.readthedocs.io/en/latest/modules/shapes.html