Huge improvements for docs (#58)

docs/content/doc/usage/api.md

@@ -0,0 +1,19 @@
+---
+date: "2019-02-12:00:00+02:00"
+title: "API Documentation"
+draft: false
+type: "doc"
+menu:
+  sidebar:
+    parent: "usage"
+---
+
+# API Documentation
+
+You can find the api docs under `http://vikunja.tld/api/v1/docs` of your instance. 
+A public instance is available on [try.vikunja.io](http://try.vikunja.io/api/v1/docs).
+
+These docs are autgenerated from annotations in the code with swagger.
+
+The specification is hosted at `http://vikunja.tld/api/v1/docs.json`.
+You can use this to embed it into other openapi compatible applications if you want.