Fix typo in plugin authoring guide.
Revise handlers page and fix some small mistakes.
Reorder and fix up some content in the handler guide.
Add an event route section to the handlers guide.
Fill out configuration page.
Update events page since some details were moved into the handlers page.
Update localization page to clarify that it is for all plugin types.
Add a section on mixins to the handlers guide.
Improve the introduction to extensions.
Fix HTML entity and minor wording on the publishing page.
Document how plugins are loaded.
Add more info about Handler#redis.
Update testing page for registry and new matchers.