Add section on representing zero repeats and add subheadings to repeat section

[lognaturel]

In preparation for XLSForm/pyxform#380 which will make it so that all repeats have a first instance.

Since this results in a change in behavior for some clients, I was at first going to talk about versions and specific clients but it got too messy:

• The XLSForm spec isn't versioned (maybe it should be? Except that there isn't really a spec, just a tutorial. It's dynamic so hard to know how we'd do versioning exactly.)
• Users generally don't even know they're using pyxform so calling out that version isn't helpful
• There are a ton of systems that use pyxform as a library so it's not reasonable to try to enumerate those versions

I think it's ok to not talk about versions and to merge this before the pyxform change is released because:

• The suggestions in the new zero repeats section are helpful and good form design even with currently-released versions of pyxform that result in Collect prompting before the first repeat
• The only known client that will change in behavior is Collect because Enketo already injects the first instance
• The change in behavior will only affect newly-converted forms
• We've decided that pyxform will generate a warning for forms that include repeats and link to this documentation so that will be the natural way for users to find this information

I included a few other perhaps controversial changes.

1. I pulled the repeat section to be a sibling of groups rather than a child. I don't think it's helpful for end users to think about repeat groups rather than just repeats. I think repeats are important enough that they merit their own section
2. I took out selects from some of the sample forms because I thought they just made the section harder to skim.
3. While I'm here, I documented how to name repeat instances. See the line before this one in the ODK XForms spec.

CC @nribeka @ukanga @MartijnR

[MartijnR]

Thanks! Looks very clear and should go a long way in avoiding support questions.

[MartijnR]

Thanks!