Make navigation pane/portal independently scrollable from main body panel.

[sync-by-unito]

The left navigation pane currently scrolls together with the main body of the topic you are currently viewing. For example if you navigate to Language -> Conventions you will find that the conventions content is larger than your current screen. If you want to maybe navigate to another topic you cannot scroll to that topic in the navigation pane without scrolling the whole page, losing context of what you were just reading. These two things should be independently scrollable.

┆Issue is synchronized with this Jira Story by Unito