added vertical margin to nav items

[whoisveroniica]

Resolves #612 by adding vertical margin to navigation items.

Margin value was picked to match jump to section margin value, mentioned in the issue.

Branch cotaines one failing test although, it also occurs in the main branch. So I decided to leave it as-is.

Below I attached screenshots showing before and after.

Desktop before	Desktop after

Mobile before	Mobile after

[davepagurek]

Somewhere we have a breakpoint adding a min-height to the main nav so that when the window height is small, we don't get scroll bars on the main nav, just on the section nav below it. Do we need to update that min height here too? Currently I believe it's 350px, might need to be increased.