Remove 'overview' items from the vertical navigation

Created by: thewilkybarkid

(For the record this change has been requested of me rather than being my idea!)

This PR removes the 'overview' links from the vertical navigation as some consider them superfluous.

So currently:

Now the overview page is gone, and the 'French' category entry is the page:

For cases where the category (ie the bold wrapper) isn't actually a page (see 'French in the MML Tripos' in the guidance), I've made it clickable when not on the first child (I'm not sure whether this is the best answer, I can see it being a bit confusing either way; this wasn't a problem before as the category was never clickable). To demonstrate this I've made 'Year 2' a page.

So, 'Year 1' shows:

And 'Year 2':