Add index reference to go template primer doc page

Add index reference to go template primer doc page

Related to @bjornerik 's answer in this discussion: http://discuss.gohugo.io/t/inserting-data-from-data-file-into-content-file-newbie-question/1002/3 ... I figured I'd make myself useful and add the reference to the index function, on the go template primer page.

Also, I moved the reference links to the bottom.

A general comment: as good as these docs are, the primer at this point makes some assumptions about audience knowledge, so some might find it lacking. Once I understand better, I might make some more clarifying edits.