add templating for title and message fields

docs/publish.md

@@ -3557,6 +3557,29 @@ ntfy server plays the role of the Push Gateway, as well as the Push Provider. Un
 !!! info
     This is not a generic Matrix Push Gateway. It only works in combination with UnifiedPush and ntfy.
 
+### Message and Title Templates
+Some services let you specify a webhook URL but do not let you modify the webhook body (e.g., Grafana). Instead of using a separate
+bridge program to parse the webhook body into the format ntfy expects, you can include a message template and/or a title template
+which will be populated based on the fields of the webhook body (so long as the webhook body is valid JSON).
+
+Send the message template with the header `X-Template-Message`, `Template-Message`, or `tpl-m`. Send the title template with the
+header `X-Template-Title`, `Template-Title`, or `tpl-t`. (No other fields can be filled with a template at this time).
+
+In the template, include paths to the appropriate JSON fields surrounded by `${` and `}`. See an example below.
+See [GJSON docs](https://github.com/tidwall/gjson/blob/master/SYNTAX.md) for supported JSON path syntax.
+
+=== "HTTP"
+    ``` http
+    POST /mytopic HTTP/1.1
+    Host: ntfy.sh
+    X-Template-Message: Error message: ${error.desc}
+    X-Template-Title: ${hostname}: A ${error.level} error has occurred
+
+    {"hostname": "philipp-pc", "error": {"level": "severe", "desc": "Disk has run out of space"}}
+    ```
+
+The example above would send a notification with a title "philipp-pc: A severe error has occurred" and a message "Error message: Disk has run out of space".
+
 ## Public topics
 Obviously all topics on ntfy.sh are public, but there are a few designated topics that are used in examples, and topics
 that you can use to try out what [authentication and access control](#authentication) looks like.