Tweak content item documentation

Clarify and make things consistent: refer to things as "content items"
and not "documents".
Rename file to better fit what it is about
Rename input example to be less ambiguous

frontend is a loaded term. This makes it clearer that this is an example
of a input for the frontend app itself.
Move format docs out of examples

These felt a bit hidden in the "examples" folders. This puts them front
and centre, making them easier to spot, and also makes the examples
folders more about just the examples (the routing.md files are to be
combined in a later commit)
Update README.md and consolidate docs

Update the README.md to do a better job of explaining what content-store is
about and how to use it. In doing so, some of the other doc files now become
redundant.