Docs improvements

docs/content/doc/development/cron.md

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+---
+title: "Cron Tasks"
+date: 2021-07-13T23:21:52+02:00
+draft: false
+menu:
+  sidebar:
+    parent: "development"
+---
+
+# How to add a cron job task
+
+Cron jobs are tasks which run on a predefined schedule.
+Vikunja uses these through a light wrapper package around the excellent [github.com/robfig/cron](https://github.com/robfig/cron) package.
+
+The package exposes a `cron.Schedule` method with two arguments: The first one to define the schedule when the cron task 
+should run, and the second one with the actual function to run at the schedule.
+You would then create a new function to register your the actual cron task in your package.
+
+A basic function to register a cron task looks like this:
+
+{{< highlight golang >}}
+func RegisterSomeCronTask() {
+	err := cron.Schedule("0 * * * *", func() {
+		// Do something every hour
+	}
+}
+{{< /highlight >}}
+
+Call the register method in the `FullInit()` method of the `init` package to actually register it.
+
+## Schedule Syntax
+
+The cron syntax uses the same on you may know from unix systems.
+
+It is described in detail [here](https://pkg.go.dev/github.com/robfig/cron#hdr-CRON_Expression_Format).