`MeshPickingPlugin` is not mentioned in the module doc

## How can Bevy's documentation be improved?

The `bevy_picking` [module doc](https://docs.rs/bevy/latest/bevy/picking/index.html) 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.