Add logstash to clients page with FrontMatter. (#2418)

Signed-off-by: Cyril Tovena <cyril.tovena@gmail.com>

docs/sources/clients/_index.md

@@ -9,7 +9,8 @@ Loki supports the following official clients for sending logs:
 - [Promtail](promtail/)
 - [Docker Driver](docker-driver/)
 - [Fluentd](fluentd/)
-- [Fluent Bit](https://github.com/grafana/loki/blob/master/cmd/fluent-bit/README.md)
+- [Fluent Bit](fluentbit/)
+- [Logstash](logstash/)
 
 ## Picking a client
 
@@ -45,11 +46,16 @@ and you already have configured `Parser` and `Filter` plugins.
 Fluentd also works well for extracting metrics from logs when using its
 Prometheus plugin.
 
+### Logstash
+
+If you are already using logstash and/or beats, this will be the easiest way to start.
+By adding our output plugin you can quickly try Loki without doing big configuration changes.
+
 # 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.
 
 - [promtail-client](https://github.com/afiskon/promtail-client) (Go)
 - [push-to-loki.py](https://github.com/sleleko/devops-kb/blob/master/python/push-to-loki.py) (Python 3)
-- [Serilog-Sinks-Loki](https://github.com/JosephWoodward/Serilog-Sinks-Loki) (C#)
+- [Serilog-Sinks-Loki](https://github.com/JosephWoodward/Serilog-Sinks-Loki) (C#)

docs/sources/clients/logstash/_index.md

@@ -1,4 +1,6 @@
-@@ -0,0 +1,269 @@
+---
+title: Logstash
+---
 # Logstash
 
 Loki has a [Logstash](https://www.elastic.co/logstash) output plugin called
@@ -272,4 +274,4 @@ output {
   }
   # stdout { codec => rubydebug }
 }
-```
+```