Moves asset import tutorial to assets section (#306)

# Description

Moves asset import tutorial to assets section as it belongs better with
the other asset tutorials instead of the environment section.

## Type of change

- This change requires a documentation update

## Screenshots

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## Checklist

- [x] I have run the [`pre-commit` checks](https://pre-commit.com/) with
`./isaaclab.sh --format`
- [x] 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
- [ ] I have added my name to the `CONTRIBUTORS.md` or my name already
exists there

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docs/source/tutorials/index.rst

@@ -43,6 +43,7 @@ class and its derivatives such as :class:`~isaaclab.assets.RigidObject`,
     :maxdepth: 1
     :titlesonly:
 
+    01_assets/add_new_robot
     01_assets/run_rigid_object
     01_assets/run_articulation
     01_assets/run_deformable_object
@@ -79,7 +80,6 @@ different aspects of the framework to create a simulation environment for agent
     03_envs/run_rl_training
     03_envs/modify_direct_rl_env
     03_envs/policy_inference_in_usd
-    03_envs/add_new_robot
 
 Integrating Sensors
 -------------------