Improve SeparationRayShape docs

[MewPurPur]

Fixes godotengine/godot-docs#8087

How did the "spears falling from the skies" come to be?

Well, I was making this massive PR refactoring physics classes' documentation, and this was a class I hadn't used before. I noticed a few issues in this page. The first was that the documentation said "SeparationRayShape is useful for characters" which seemed so broad. You can use everything for characters except for stuff like worldboundaries and heightmaps. So I started thinking about how to give a more obvious example.

The second issue in the old documentation page, and it was a reoccuring issue, was a performance remark along the lines of "SeparationRayShape is fast, but CircleShape is faster". Again, why would you mention this? It's not like the two shapes are similar to any capacity, no one will replace their SeparationRayShapes for CircleShapes for performance reasons.

Wanting to make these performance remarks more useful, I made a test project and started spawning a lot of physics shapes. Spawning 1000 SeparationRayShapes looked like a "Rain of spears" sorta spell, so it gave me the idea to put this as an example in the docs. I didn't even look into its common use cases 😓

[akien-mga]

Thanks!