Fixes nested dropdowns for tutorial documentation (#295)

# Description

Creates nested dropdowns in the sidebar for Tutorials. I also eliminated
the redundant titles and placed the the links above their respective
descriptions on the page clicking Tutorials directly returns. Aesthetic
NITs welcome! 😄

## Type of change

- New feature (non-breaking change which adds functionality)

## Checklist

- [x] I have run the [`pre-commit` checks](https://pre-commit.com/) with
`./orbit.sh --format`
- [ ] I have made corresponding changes to the documentation
- [x] My changes generate no new warnings
- [ ] I have added tests that prove my fix is effective or that my
feature works
- [ ] I have updated the changelog and the corresponding version in the
extension's `config/extension.toml` file
- [x] I have added my name to the `CONTRIBUTORS.md` or my name already
exists there

docs/source/tutorials/00_sim/index.rst

+Setting up a Simple Simulation
+==============================
+.. toctree::
+    :maxdepth: 1
+    :titlesonly:
+
+    launch_app
+    create_empty
+    spawn_prims

docs/source/tutorials/01_assets/index.rst

+Interacting with Assets
+=======================
+.. toctree::
+    :maxdepth: 1
+    :titlesonly:
+
+    run_articulation
+    run_rigid_object

docs/source/tutorials/02_scene/index.rst

+Creating a Scene
+================
+.. toctree::
+    :maxdepth: 1
+    :titlesonly:
+
+    create_scene

docs/source/tutorials/03_envs/index.rst

+Designing an Environment
+========================
+.. toctree::
+    :maxdepth: 1
+    :titlesonly:
+
+    create_base_env
+    create_rl_env
+    register_rl_env_gym
+    run_rl_training

docs/source/tutorials/04_sensors/index.rst

+Integrating Sensors
+===================
+.. toctree::
+    :maxdepth: 1
+    :titlesonly:
+
+    add_sensors_on_robot
+    run_frame_transformer
+    run_ray_caster
+    run_ray_caster_camera
+    run_usd_camera

docs/source/tutorials/05_controllers/index.rst

+Using Motion Generators
+=======================
+.. toctree::
+    :maxdepth: 1
+    :titlesonly:
+
+    ik_controller

docs/source/tutorials/index.rst

@@ -13,9 +13,12 @@ the ``source/standalone/tutorials`` directory of the Orbit repository.
     We would love to extend the tutorials to cover more topics and use cases, so please let us know if
     you have any suggestions.
 
+.. toctree::
+   :maxdepth: 1
+   :titlesonly:
+
+   00_sim/index
 
-Setting up a Simple Simulation
-------------------------------
 
 These tutorials show you how to launch the simulation with different settings and spawn objects in the
 simulated scene. They cover the following APIs: :class:`~omni.isaac.orbit.app.AppLauncher`,
@@ -25,12 +28,8 @@ simulated scene. They cover the following APIs: :class:`~omni.isaac.orbit.app.Ap
     :maxdepth: 1
     :titlesonly:
 
-    00_sim/launch_app
-    00_sim/create_empty
-    00_sim/spawn_prims
+    01_assets/index
 
-Interacting with Assets
------------------------
 
 Having spawned objects in the scene, these tutorials show you how to create physics handles for these
 objects and interact with them. These revolve around the :class:`~omni.isaac.orbit.assets.AssetBase`
@@ -41,11 +40,8 @@ class and its derivatives such as :class:`~omni.isaac.orbit.assets.RigidObject`
     :maxdepth: 1
     :titlesonly:
 
-    01_assets/run_rigid_object
-    01_assets/run_articulation
+    02_scene/index
 
-Creating a Scene
-----------------
 
 With the basic concepts of the framework covered, the tutorials move to a more intuitive scene
 interface that uses the :class:`~omni.isaac.orbit.scene.InteractiveScene` class. This class
@@ -55,10 +51,8 @@ provides a higher level abstraction for creating scenes easily.
     :maxdepth: 1
     :titlesonly:
 
-    02_scene/create_scene
+    03_envs/index
 
-Designing an Environment
-------------------------
 
 The following tutorials introduce the concept of environments: :class:`~omni.isaac.orbit.envs.BaseEnv`
 and its derivative :class:`~omni.isaac.orbit.envs.RLTaskEnv`. These environments bring-in together
@@ -68,36 +62,19 @@ different aspects of the framework to create a simulation environment for agent
     :maxdepth: 1
     :titlesonly:
 
-    03_envs/create_base_env
-    03_envs/create_rl_env
-    03_envs/register_rl_env_gym
-    03_envs/run_rl_training
+    04_sensors/index
 
-Integrating Sensors
--------------------
 
 The following tutorials show you how to integrate sensors into the simulation environment. These
 tutorials introduce the :class:`~omni.isaac.orbit.sensors.SensorBase` class and its derivatives
 such as :class:`~omni.isaac.orbit.sensors.FrameTransformer` and :class:`~omni.isaac.orbit.sensors.RayCaster`.
 
+
 .. toctree::
     :maxdepth: 1
     :titlesonly:
 
-    04_sensors/add_sensors_on_robot
-    04_sensors/run_frame_transformer
-    04_sensors/run_ray_caster
-    04_sensors/run_ray_caster_camera
-    04_sensors/run_usd_camera
-
-Using Motion Generators
------------------------
+    05_controllers/index
 
 While the robots in the simulation environment can be controlled at the joint-level, the following
 tutorials show you how to use motion generators to control the robots at the task-level.
-
-.. toctree::
-    :maxdepth: 1
-    :titlesonly:
-
-    05_controllers/ik_controller