document the order in which hooks are executed

[nilslice]

there was an issue discussed in the #ponzu slack channel where knowing the order in which content type's item.Hookable methods are executed may have helped a lot. this helps users determine the state of the database and what is available at what time.

documentation would be great to mention this order (note the hook invocations): https://github.com/ponzu-cms/ponzu/blob/ec7b5b2bab2cad13db317a017fb8ee5d32a8f68c/system/admin/handlers.go#L1619-L1710

these are not the only hooks called. other handler funcs have the other item.Hookable methods. those should be documented as well.

the documentation should live in https://github.com/ponzu-cms/docs and copied into https://github.com/ponzu-cms/ponzu