Restructure of the docs content

docs/advanced/index.md

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+---
+has_children: false
+nav_order: 3
+---
+
+# Advanced
+
+In this section we will add the documentation for all subjects that go beyond
+the beginner topics, but which are interesting for mission builders or power
+users.
+
+{: .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