Add Cache-Control "public,max-age=0,must-revalidate" to index.html in SPA example

Add the Cache-Control "public,max-age=0,must-revalidate" for index.html header to the SPA example.

In SPAs the index.html must be prevented from being cached, in the index.html the (hashed) javascript and css file are referenced (these can be cached becaused they get a new random suffix everytime they are built)

src/docs/markdown/caddyfile/patterns.md

@@ -218,7 +218,12 @@ A typical SPA config usually looks something like this:
 example.com {
 	root * /srv
 	encode gzip
-	try_files {path} /index.html
+	route {
+		try_files {path} /index.html
+		header /index.html {
+			Cache-Control "public,max-age=0,must-revalidate"
+		}
+	}
 	file_server
 }
 ```