doc: search terms

[hkershaw-brown]

I find when I am searching for phrases such as 'time manager' I don't get the page I am looking for because I'm not using underscores:
-> need to search "time_manager_mod"

I think adding index terms for a page for search terms would be super helpful for me, and probably users too, e.g. to time_manager_mod.rst add:

.. index:: time manager, time_manager_mod

I think this could be helpful for directing people, e.g. should someone searching 'OSSE' get perfect model obs in the search results?

[hkershaw-brown]

Another example: searching qty or quantity does not get you to obs_kind_mod at the moment.