docs/index: merge analog, digital and simulation toctrees

Signed-off-by: Unai Martinez-Corral <umartinezcorral@antmicro.com>

Author: umarcor

docs/index.rst

@@ -67,27 +67,13 @@ Table of contents
 
 
 .. toctree::
-    :caption: Analog Design
+    :caption: Design and Simulation
     :name: analog
     :maxdepth: -1
 
-    analog/index
-
-
-.. toctree::
-    :caption: Digital Design
-    :name: digital
-    :maxdepth: -1
-
-    digital/index
-
-
-.. toctree::
-    :caption: Simulation
-    :name: sim
-    :maxdepth: -1
-
-    sim/index
+    Analog Design <analog/index>
+    Digital Design <digital/index>
+    Simulation <sim/index>
 
 
 .. toctree::