Add [Index] link to package front page
Add [Index] link to package front page

If there is documentation for a package, display an "[Index]" link below
the list of modules, linking directly to the documentation index.

This is similar to #196, except the link is placed in the page body (as
per @dcoutts) rather than the header.

Closes #187.

agrafix/hackage-server,erantapaa/hackage-server,snoyberg/hackage-server,snoyberg/hackage-server,erantapaa/hackage-server,dzackgarza/hackage-server,grayjay/hackage-server,dzackgarza/hackage-server,edsko/hackage-server,ocharles/hackage-server,agrafix/hackage-server,chrisdotcode/hackage-server,tkvogt/hackage-server,grayjay/hackage-server,ocharles/hackage-server,chrisdotcode/hackage-server,haskell-infra/hackage-server,tkvogt/hackage-server,edsko/hackage-server