Adds current environments to the docs (#302)

# Description

Adds current environments to the docs with a description, an image, and
a link to the environment.

## 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`
- [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
- [x] I have added my name to the `CONTRIBUTORS.md` or my name already
exists there

docs/index.rst

@@ -34,7 +34,7 @@ For more information about the framework, please refer to the `paper <https://ar
 
    source/features/environments
    source/features/actuators
-   source/features/motion_generators
+   .. source/features/motion_generators
 
 .. toctree::
    :maxdepth: 1