Documentation Revamp (#496)

Added a new documentation structure with details of concepts, tutorials and examples

Documentation/docs-site/content/concepts/trigger.en.md

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+---
+title: "Trigger"
+draft: false
+weight: 34
+---
+
+Triggers are events that can invoke a [function](../function). Fission has three kinds of triggers that can be used to invoke functions.
+
+## Http Trigger
+
+HTTP triggers enable calling functions with HTTP requests. Supported methods are GET, POST, PUT, DELETE, HEAD and by default GET is used. URL pattern follow the gorilla/mux supported patterns.
+
+## Time Trigger
+
+If you want a function to be called at a periodic frequency then the time triggers are perfect for the use case. Time triggers follow cron like specifications and are invoked based on the cron schedule.
+
+Time trigger based invocations are great for running scheduled jobs, periodic cleanup jobs, periodic polling based invocations etc. 
+
+## MQ Trigger
+
+Message queue based trigger enables ability to listen on a topic and invoke a function for each message. You can optinally send a response to another topic. By default it is assumed that the messages in queue are in application/json format but you can specify otherwise while creating the trigger. Currently `nats-streaming` and `azure-storage-queue` are supported message queues supported.
+
+MQ triggers are great for integrating various systems in a decoupled and asynchronous manner.