Clarify partials folder subfolders

Clarify partials folder subfolders

A couple of edits to clarify that the layout/partials folder can contain arbitrarily-named subfolders, since I found the examples using ``{{ partial "post/tag/list" . }}`` confusing. Some folders are named specifically to work a certain way with hugo, but although the examples use key functional section and taxonomy names like post and tag, it does not matter what they are called. Hopefully this will help other newbs.