Restricts .gitignore rule to top-level datasets/ directory (#3400)

# Description

As noted by @liyifan2002 in the related issue, the `.gitignore` rule for
`datasets` also unintentionally ignores changes in
`isaaclab/utils/datasets/`.

This PR fixes the problem by changing the rule from `datasets` to
`/datasets/`, ensuring that only the top-level `datasets/` folder (e.g.,
created when following [this
guide](https://isaac-sim.github.io/IsaacLab/main/source/overview/imitation-learning/skillgen.html#download-and-setup))
is ignored, while keeping `isaaclab/utils/datasets/` tracked.

Fixes #3364

## Type of change

- Bug fix (non-breaking change which fixes an issue)

## Checklist

- [x] I have run the [`pre-commit` checks](https://pre-commit.com/) with
`./isaaclab.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

Author: louislelay

.gitignore

@@ -62,7 +62,7 @@ _build
 /.pretrained_checkpoints/
 
 # Teleop Recorded Dataset
-datasets
+/datasets/
 
 # Tests
 tests/