Document overriding mailer templates (#10574)

* Document overriding mailer templates

* List emails in backend guide index
Issue #9849

docs/backend/emails.md

@@ -0,0 +1,13 @@
+---
+title: Emails
+---
+
+# Previewing emails in development
+
+You can view email previews at <http://localhost:3000/rails/mailers>.
+
+Previews are setup in the directory `spec/mailers/previews`.
+
+# Overriding mailer templates
+
+To update the contents of emails that the app sends, edit the views under `app/views/mailers`. Note that this app uses the [`devise_invitable` gem](https://github.com/scambra/devise_invitable) for invitations. The views for this gem are stored under `app/views/devise/mailer`.

docs/backend/previewing-emails.md

@@ -1,9 +0,0 @@
----
-title: Previewing Emails
----
-
-# Previewing emails in development
-
-You can view email previews at <http://localhost:3000/rails/mailers>.
-
-Previews are setup in the directory `spec/mailers/previews`.

docs/backend/readme.md

@@ -10,12 +10,12 @@ items:
   - configuration.md
   - data-update-scripts.md
   - elasticsearch.md
+  - emails.md
   - fastly.md
   - internationalization.md
   - roles.md
   - pusher.md
   - resource-admin.md
-  - previewing-emails.md
   - notification.md
   - scheduled-jobs.md
   - metrics.md