Refactor at_plus examples - and others - into useful HOWTO guide docs on querying & indexing

DOC-2190 Need to explain how to use at_plus for each SDK gave us code examples for C, Java, and dot NET.

These need to be turned into HOWTO docs that show users how to get something useful out of  indexes, rather than just show a piecemeal chunk of code.

Without overloading the HOWTO concept, this can be a longer piece of code - explained step-by-step - which shows how to put together some of the basic connecting, querying, and working with indexes concepts only shown piecemeal in the current docs.

This may split naturally into a number of HOWTOs.