`MeshPickingPlugin` is not mentioned in the module doc

[djeedai]

How can Bevy's documentation be improved?

The bevy_picking module doc doesn't mention anywhere the MeshPickingPlugin, which is critical not only for 3D meshes but also for 2D ones, because:

• that plugin is not added by default
• 2D meshes are different from sprites, so require the MeshPickingPlugin

The result is that spawning a Mesh2d + PickingInteraction does nothing, and the user can't understand why from docs.

In fact, even for 3D meshes, that top-level module documentation never mentions this plugin, despite the fact the doc does refer to mesh picking.