Add missing documentation for extension hooks
This change adds docs for the following hooks:

* ActionHook (and its various subclasses)

* FileAttachmentThumbnailHook

* ReviewRequestDetailHook

* TemplateHook
I've arranged the hooks documentation alphabetically, since I'm not sure of a

better way. While I was doing this I noticed a little bit of weirdness in the

ActionHook init (actions should have been an empty list, not an empty

dict).

Generated the HTML docs and checked that things made sense and cross-linked

correctly.