Put categories pages in a subdirectory of each doc type.

Refs #9678
Move functions to fitfunctions

Refs #9678
Move functions to FitFunctions category.

Refs #9678
Write out index.html pages for a pre-defined set of categories.

Refs #9678
Update main index page and add api index page.

Refs #9678
Remove custom styling for description list tags.

The standard spacing seems just fine.
Refs #9678
Force everything to be rebuilt on incremental builds

The Category objects in the categories directive have been pickled so
changing the structure can lead to problems for incremental builds.
Refs #9678
Merge remote-tracking branch 'origin/feature/9678_docs_index_pages'