Improve search behavior for specific keywords

[mppf]

This PR improves the documentation search results for specific searches:

• new
• with
• by
• zip
• local

In order to do so, it adds .. index:: entries to the forallLoops.chpl and taskParallel.chpl primers. I added index entries for other sections present there, while there. To get these .. index:: entries to work, I needed to update literate_chapel.py because it was de-indenting the lines following .. index:: which lead to invalid RST.