Add link to travel insurance benefit from the travel page.

Searching for the (fairly common) term "travel insurance" in the handbook didn't get me very good results, so

1. I've added a link from the travel page to details of the travel insurance benefit.
2. I've specifically used the term "travel insurance".

@Tknudsen I've assigned you just in case there are any legal gotchas with the specific terminology I've used.