enhance documentation

- flatten hierarchy (some links do not work when using folders)
- add a tutorial for programming a simple driver
- clean description using inspect.cleandoc
+ fix a bug with 'unit' pseudo property in a Parameter used as override

Change-Id: I31ddba5d516d1ee5e785e28fbd79fca44ed23f5e
Reviewed-on: https://forge.frm2.tum.de/review/c/sine2020/secop/playground/+/25000
Tested-by: Jenkins Automated Tests <pedersen+jenkins@frm2.tum.de>
Reviewed-by: Markus Zolliker <markus.zolliker@psi.ch>

doc/source/index.rst

@@ -1,19 +1,16 @@
-Welcome to FRAPPY documentation!
-================================
+Frappy Programming Guide
+========================
 
 .. toctree::
    :maxdepth: 2
 
-   server/index
-   client/index
-   framework/index
-   gui/index
-   facility/index
-
-
-Indices and tables
-==================
+   introduction
+   tutorial
+   reference
+   secop_psi
+   secop_demo
+   secop_mlz
+   secop_ess
 
 * :ref:`genindex`
-* :ref:`modindex`
-* :ref:`search`
+