Make Sidebar index follows (scroll to) the current visible page

[YahiaJr]

When viewing a section in the wiki that's in the lower section in the index, it doesn't appear by default in the sidebar.

for example, when viewing the nix-related instructions, every time I need to go to another page (not the next one necessarily) in the nix section I have to scroll down to be able to see the rest of the section's pages.

it's not much of an issue, but it's inconvenient UX.