refactor(docs): index: do 'what to find where'

docs/index.md

@@ -16,8 +16,31 @@ The source code is publicly available from the [transfer portal][transferportal]
 
 ## What to find where
 
-!!! todo
-    explain how the documentation is structured
+The documentation is split in three main parts, navigable on the top-bar.
+Each main-part gives you a navigation of it's content on the left.
+On the right, the currents page table of contents is shown.  
+The icon in the upper left brings you from anywhere back to this start-page.
+In the upper right you can go to the source code repository of this library and documentation.
+Leftwards a full-text search is provided.
+
+The part you are reading right now (__Home__), is the main website.
+Following is a list, what each site contains:
+
+ - __Changelog:__ auto-generated changelog from the commits
+ - __Contributing:__
+     - Prerequisites to be able to be a contributor to this project
+     - Needed machine-setup to develop (both for the library and documentation part)
+ - __License:__ GNU GPLv3 content
+ - __Usage:__
+     - needed entries to get this library into your own maven-project
+     - some code-snippets showing the libs capabilites
+
+The __API__ part holds a detailed description, for each publicly accessible class and their methods.
+There is an own index file, explaining the package structure of the library
+and how it is documented.
+
+The __Developers Guide__ is the part, you have to lookup the conventions and workflows used in this project,
+if you decide to contribute to it.
 
 ## Maintainer