Break down the Widgets user guide

- A how-to for groupby (something like how to explore a multidimensional dataset with widgets)
- Copy/paste (and adapt a bit) the part of the Widgets user guide that reference Panel widgets explicitly to the Reference section. This is API that we would like to revisit and I don't want to expose it too much in a Tutorial, but also don't want to lose that content.
- Nice to have: People are confused between groupby (HoloMap/DynamicMap containers), by (NdOverlays or NdLayouts if subplots=True), and color/c for vectorized. These options have two main outputs. First the HoloViews object returned is different and can be re-used (e.g. indexed) differently, e.g. `by='sex'` will return an NdOverlay that can be indexed by sex (`plot['female']`) while `color='sex'` returns a single Scatter element that can't be indexed the same way. Second, the visual output can differ or not. And a last one, there can be some performance benefits to use `color` over `by`. All of this would be nice to document in an Explanation page.