Merge pull request #278 from RobotControlStack/docs/structure-cleanup

docs: clean up docs structure and navigation

Author: juelg

docs/index.md

@@ -18,6 +18,14 @@
 
 ## Documentation
 
+```{toctree}
+:maxdepth: 2
+:caption: User Guide
+
+getting_started/index
+user_guide/index
+```
+
 ```{toctree}
 :maxdepth: 2
 :caption: API

docs/user_guide/index.md

@@ -0,0 +1,11 @@
+# User Guide
+
+The User Guide provides in-depth information about the core concepts and components of the Robot Control Stack.
+
+```{toctree}
+:maxdepth: 2
+
+architecture
+gym_interface
+low_level_api
+```