[v9.4.x] Docs: Add example to extract a specific label value (#66018)

Docs: Add the index function

(cherry picked from commit a39190b613)

Co-authored-by: Simon Crute <simonc6372@users.noreply.github.com>

docs/sources/alerting/manage-notifications/template-notifications/using-go-templating-language.md

@@ -71,6 +71,16 @@ The name of the annotation is {{ .Name }}, and the value is {{ .Value }}
 {{ end }}
 ```
 
+## The index function
+
+To print a specific annotation or label use the `index` function.
+
+```
+{{ range .Alerts }}
+The name of the alert is {{ index .Labels "alertname" }}
+{{ end }}
+```
+
 ## If statements
 
 You can use if statements in templates. For example, to print `There are no alerts` if there are no alerts in `.Alerts` you would write the following: