Re-org (re-think) form building section

This is the most disjointed and disorganized section of the docs.
The Intro to Forms page helped a lot, but it's still kind of a mess.

It would probably help to divide it into sections.

Possibly:

- XForm
- XLSForms
- Forms in Collect <--- Not sure what to call this

I have to think about this some more. Opinions welcome.

Start by reviewing the existing Intro to Forms page and the surrounding form-building documentation. Compare the proposed XForm, XLSForms, and Forms in Collect groupings, then define a settled section structure and update the documentation so users can find each topic consistently.

Written by the indexing model from the issue text.