Start to add afform docs

Note that our 'best' documentation on form-builder at the moment is https://lab.civicrm.org/extensions/afform/-/tree/master which is very out of date. My goal here is mostly to establish a place to start documenting it.

I only copied over one existing md file as when I got to the second one it seemed out-of-date

@totten @MikeyMJCO @seamuslee @colemanw