Add AnchorJS to aid with linking to page fragments

After trying to manually create a link to a page section for the nth time,
I finally scratched that itch and added AnchorJS to provide the links for
us. This greatly simplifies the process for linking to sections of pages,
especially useful for those not versed in digging around in the page
source.

Signed-off-by: Martyn Welch <martyn.welch@collabora.com>