Better getting started page for form design

AT http://docs.opendatakit.org/form-design-intro and http://docs.opendatakit.org/xlsform, it's odd that we link to xlsform.org as the place to learn how to design a form when we have a bunch of great form design docs. Perhaps once those docs are complete, we can have an overview or getting started page about XLSForm?

Start by reading the form-design-intro and xlsform pages, then review the existing form design documentation they reference. The work is done when the documentation provides a coherent XLSForm getting-started or overview page and no longer directs learners to xlsform.org as the primary place to learn form design.

Written by the indexing model from the issue text.