[feature req] Dissociate scrolling of left and right side parts

[juanlanus]

These are a few suggestions on how I would implement the docs UI, based on my experience doing UX, or usability, you name it.
I remark that this is whet I would do, others might have better ideas.

While browsing the docs, the left menu col is the context, and the right hand bigger pane is the content.
Ideally the reader could switch her focus back and forth having a stable context.
As I see it, the context should change only under the user's control, and ideally using animations that give him time to notice what is happening and where. Abrupt changes convey a disturbing sense of "what did I break this time?!".

Another related suggestion is when the user clicks a menu item and it opens in the left-hand column but nothing happens in the right pane, like in the examples where the user is required to click [_starter] to get into.
For these cases when the menu just opens, is was customary to provide sort of a landing page (possibly blank, or with just a title, ideally with the list of the just-disclosed menu items with short descriptions added) so the user experiments the expected behavior: click the left to change the right. Clicking the menu and nothing happening on the right conveys a slight feeling of frozen UI.

One issue scroll dissociation fixes is the vertical scroll needed when there are many menu groups disclosed and the selected content appears in the (not visible) upper part of the content pane.
I thought the item was broken, until after a while I realized it was there but not visible.

Another advantage of the scroll dissociation would be to fix the unexpected appearance of a vertical scrollbar to the right of the viewport after opening the [> grommet] menu item in https://grommet-nextjs.herokuapp.com/examples.
In fact, that scrollbar actually belongs to the left hand column, the right content pane is innocent.

I like this pages and I think they are going to be quite useful. Additionally, I realize that this is very new, in fact, the best time to introduce changes. Congrats!