Add Missing Email configs in Configuration doc (#13709)

closes https://github.com/apache/airflow/issues/13697

airflow/config_templates/config.yml

@@ -1262,6 +1262,24 @@
       type: boolean
       example: ~
       default: "True"
+    - name: subject_template
+      description: |
+        File that will be used as the template for Email subject (which will be rendered using Jinja2).
+        If not set, Airflow uses a base template.
+      version_added: ~
+      type: string
+      example: "/path/to/my_subject_template_file"
+      default: ~
+      see_also: ":doc:`Email Configuration </howto/email-config>`"
+    - name: html_content_template
+      description: |
+        File that will be used as the template for Email content (which will be rendered using Jinja2).
+        If not set, Airflow uses a base template.
+      version_added: ~
+      type: string
+      example: "/path/to/my_html_content_template_file"
+      default: ~
+      see_also: ":doc:`Email Configuration </howto/email-config>`"
 
 - name: smtp
   description: |

airflow/config_templates/default_airflow.cfg

@@ -626,6 +626,16 @@ default_email_on_retry = True
 # Whether email alerts should be sent when a task failed
 default_email_on_failure = True
 
+# File that will be used as the template for Email subject (which will be rendered using Jinja2).
+# If not set, Airflow uses a base template.
+# Example: subject_template = /path/to/my_subject_template_file
+# subject_template =
+
+# File that will be used as the template for Email content (which will be rendered using Jinja2).
+# If not set, Airflow uses a base template.
+# Example: html_content_template = /path/to/my_html_content_template_file
+# html_content_template =
+
 [smtp]
 
 # If you want airflow to send emails on retries, failure, and you want to use