[docs/clients] fix header for "Unofficial clients" (#3720)

and add a reference to said section.

docs/sources/clients/_index.md

@@ -13,6 +13,8 @@ Loki supports the following official clients for sending logs:
 - [Logstash](logstash/)
 - [Lambda Promtail](lambda-promtail/)
 
+There are also a number of third-party clients, see [Unofficial clients](#unofficial-clients).
+
 ## Picking a client
 
 While all clients can be used simultaneously to cover multiple use cases, which
@@ -58,7 +60,7 @@ This is a workflow combining the promtail push-api [scrape config](promtail/conf
 
 This is a good choice if you're looking to try out Loki in a low-footprint way or if you wish to monitor AWS lambda logs in Loki.
 
-# Unofficial clients
+## Unofficial clients
 
 Please note that the Loki API is not stable yet, so breaking changes might occur
 when using or writing a third-party client.