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):

ponzu/system/admin/handlers.go

Lines 1619 to 1710 in ec7b5b2

	// run hooks
	hook, ok := post.(item.Hookable)
	if !ok {
	log.Println("Type", t, "does not implement item.Hookable or embed item.Item.")
	res.WriteHeader(http.StatusBadRequest)
	errView, err := Error400()
	if err != nil {
	return
	}
	res.Write(errView)
	return
	}
	// check if we have a Mergeable
	m, ok := post.(editor.Mergeable)
	if !ok {
	log.Println("Content type", t, "must implement editor.Mergeable before it can be approved.")
	res.WriteHeader(http.StatusBadRequest)
	errView, err := Error400()
	if err != nil {
	return
	}
	res.Write(errView)
	return
	}
	dec := schema.NewDecoder()
	dec.IgnoreUnknownKeys(true)
	dec.SetAliasTag("json")
	err = dec.Decode(post, req.Form)
	if err != nil {
	log.Println("Error decoding post form for content approval:", t, err)
	res.WriteHeader(http.StatusInternalServerError)
	errView, err := Error500()
	if err != nil {
	return
	}
	res.Write(errView)
	return
	}
	err = hook.BeforeApprove(res, req)
	if err != nil {
	log.Println("Error running BeforeApprove hook in approveContentHandler for:", t, err)
	return
	}
	// call its Approve method
	err = m.Approve(res, req)
	if err != nil {
	log.Println("Error running Approve method in approveContentHandler for:", t, err)
	return
	}
	err = hook.AfterApprove(res, req)
	if err != nil {
	log.Println("Error running AfterApprove hook in approveContentHandler for:", t, err)
	return
	}
	err = hook.BeforeSave(res, req)
	if err != nil {
	log.Println("Error running BeforeSave hook in approveContentHandler for:", t, err)
	return
	}
	// Store the content in the bucket t
	id, err := db.SetContent(t+":-1", req.Form)
	if err != nil {
	log.Println("Error storing content in approveContentHandler for:", t, err)
	res.WriteHeader(http.StatusInternalServerError)
	errView, err := Error500()
	if err != nil {
	return
	}
	res.Write(errView)
	return
	}
	// set the target in the context so user can get saved value from db in hook
	ctx := context.WithValue(req.Context(), "target", fmt.Sprintf("%s:%d", t, id))
	req = req.WithContext(ctx)
	err = hook.AfterSave(res, req)
	if err != nil {
	log.Println("Error running AfterSave hook in approveContentHandler for:", t, err)
	return
	}

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