improved doxygen documentation

doc/main.dox

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+/**
+
+    @mainpage
+
+    This is the entry to the doxygen documentation of the shellmatta library.
+
+    Please find the documenation here:
+
+    @subpage shellmatta
+
+*/

doc/shellmatta.dox

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+/**
+
+    @page shellmatta Shellmatta
+
+    The shellmatta is a tiny shell implementation to be integrated in all kinds
+    of software projects to add a debug and configuration interface.
+
+    Please take a look at the README.md file for some information that might
+    not be included here.
+
+    @section shellmatta_api_section Shellmatta api description
+
+    The complete api of the shellmatta is included in the file api/shellmatta.h.
+
+    The api description can be found here:
+    @subpage shellmatta_api
+
+    This is how the classic usage looks like:
+
+    @startuml
+        App -> Shellmatta: shellmatta_doInit()
+
+        loop for every command
+            App -> Shellmatta: shellmatta_addCmd(command)
+        end
+
+        loop until finished
+            IO -> Shellmatta: shellmatta_processData(data)
+            Shellmatta -> App: call command function
+            App -> Shellmatta: shellmatta_printf(output data)
+            Shellmatta -> IO: write(data)
+        end
+    @enduml
+
+*/