Follow-up work on 2.1 docs sidenav

[jseldess]

As a follow-up to #3270:

• We should make the sidenav independently scrollable.
• We should fix this: When you select a page in the sidenav, the sidenav collapses to the page's parent. So the selected page isn't visible. See example below.

Example:

It should look like:

[jseldess]

Closed by #3284.