Validator class workflows should be mentioned in usage docs

[cdunklau]

I was looking for a way to do what jsonschema.validators.validator_for does, but didn't find it in the initial "basic usage" section. A band-aid could be to expand the basic usage documentation (say, where it points out "If you know you have a valid schema already...").

...however I must say I was confused by the layout of the docs. The table of contents seems to imply a more narrative style, but the docs intersperse prose with generated API documentation. I think a cleaner separation of narrative and API docs (with appropriate cross-references) could be more friendly and effective.