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.
Merge pull request #209 from lfairy/doc-index-link

Add [Index] link to package front page