Add links to the field guide index

This commit takes the subsection summary in the field guid index
and adds internal links to the field guide page. Previously when
viewing the field guide docs it wasn't clear how to navigate between
all the different test class pages, this is remedied by adding a link.

Change-Id: Ica93a92c57e5d5d805a8b25d7c7e17509695152b
Merge "Add links to the field guide index"