Restructure of the docs content

docs/developer/index.md

@@ -0,0 +1,21 @@
+---
+title: Developer
+has_children: true
+nav_order: 4
+---
+
+# Developer
+
+All information in this section is primarily intended for developers who extend
+or maintain the Moose program code. Of course, this information may also be of
+interest to others who want to learn more about the background of MOOSE.
+
+{: .note }
+> This documentation is WIP (work in progress) and it will take some time to
+> fill it with usefull and up to date informations. Please be patient and check
+> back here from time to time to see if there is anything new for you.
+
+If you cannot find the information you are looking for here in the new
+documentation, please take a look at the [archive].
+
+[archive]: ../archive/index.md