Add/improve documentation

[lkubb]

The process of creating and maintaining Salt extensions (within or outside of this organization) is currently under-documented. I have read several accounts that describe difficulty and confusion around it - the lack of community maintainers might be a symptom.

Since this Copier template could serve as the pivot of most Salt extension development, this repository would be a natural choice for general documentation/guides regarding

a) creating
b) writing + testing
c) documenting
d) publishing
e) maintaining

Salt extensions (at least for a start, we could outsource it later once it's more established).

I'd like to include a proper, Sphinx-built documentation regarding these topics.

Related: #15