doc and clean up bevy_shader

[atlv24]

Objective

• murder mwriting monday's mortal menemy #![expect(missing_docs, reason = "Not all docs are written yet, see #3492.")]
• clean up bevy_shader

Solution

• move RenderDevice param from get to new, because we will soon be in a Render Recovery world and need to ensure different render devices do not end up in the same shader cache.
• delete an unused enum
• delete some boilerplate getters
• document everything
• also document define_atomic_id macro

Testing

• ci