[#943] Add cross-ref from authorization docs to IAuthFunctions
[#943] Minor tweaks to the writing extensions tutorial
[#943] Move some of the writing extensions tutorial sections into separate files
[#943] Remove the Publishing extensions page from the docs

It wasn't very good
[#943] Typo
[#943] Tweak the plugin interfaces reference docs
[#943] Tweak get_action() docstring
[#943] Add validator functions reference to docs

These are available to plugins to toolkit.get_validator(), so they need
to be in the docs.
[#943] Tweak plugin interfaces reference docs