Fix PartialCommit, fix module references, link to sub-pages from the index, and tidying

At the moment https://collectivejsonmigrator.readthedocs.org/ provides no way to navigate to sub-pages, so remove ``hidden`` directive from TOC. 
Grammar & spelling fixes.
Move `PartialCommit` from `blueprints` to its own module (other blueprint steps still need to be done).