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>

secop/lib/asynconn.py

@@ -126,7 +126,7 @@ class AsynConn:
             self._rxbuffer += data
 
     def readbytes(self, nbytes, timeout=None):
-        """read one line
+        """read a fixed number of bytes
 
         return either <nbytes> bytes or None if not enough data available within 1 sec (self.timeout)
         if a non-zero timeout is given, a timeout error is raised instead of returning None