Add some placeholders

_sidebar.md

@@ -8,8 +8,10 @@
 - [Ruby](categories/ruby.md)
 - [Interview](categories/interviews/interview.md)
 - [Software Design](categories/software-design.md)
-- [Queues](categories/queues.md)
-    - [Kafka](categories/queues/kafka.md)
+- [Apps and Tools](categories/apps.md)
+    - [Kafka](categories/apps/kafka.md)
+    - [MQTT](categories/apps/mqtt.md)
+    - [Elasticsearch](categories/apps/elasticsearch.md)
 - [Database](categories/database.md)
 - [Application](categories/application.md)
 - [Web Scraping](categories/web-scraping.md)

categories/apps.md

@@ -0,0 +1 @@
+# Some apps and tools

categories/apps/elasticsearch.md

@@ -0,0 +1,3 @@
+# Elasticsearch
+
+Elasticsearch is a search engine based on the Lucene library. It provides a distributed, multitenant-capable full-text search engine with an HTTP web interface and schema-free JSON documents.

categories/apps/mqtt.md

@@ -1,11 +1,3 @@
-# Message Queue
-
-## Rabbitmq
-
-## activeMQ
-
-## Redis
-
 ## MQTT
 
 A lightweight messaging protocol for small sensors and mobile devices, optimized for high-latency or unreliable networks. It's basically a machine-to-machine `protocol`