Adds check for .gitignore when generating template (#575)

# Description

When generating a new template from docker, .gitignore may not be
present and causes an error in the generator tool when it tries to copy
the .gitignore file into the template project. This change adds a check
for if .gitignore is present before attempting to copy it.


## Type of change

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applicable. -->

- 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`
- [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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putting an x character in it

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- [x] I have done this task
- [ ] I have not done this task
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tools/template/generator.py

@@ -163,6 +163,7 @@ def _external(specification: dict) -> None:
     shutil.copyfile(os.path.join(ROOT_DIR, ".dockerignore"), os.path.join(project_dir, ".dockerignore"))
     shutil.copyfile(os.path.join(ROOT_DIR, ".flake8"), os.path.join(project_dir, ".flake8"))
     shutil.copyfile(os.path.join(ROOT_DIR, ".gitattributes"), os.path.join(project_dir, ".gitattributes"))
+    if os.path.exists(os.path.join(ROOT_DIR, ".gitignore")):
         shutil.copyfile(os.path.join(ROOT_DIR, ".gitignore"), os.path.join(project_dir, ".gitignore"))
     shutil.copyfile(
         os.path.join(ROOT_DIR, ".pre-commit-config.yaml"), os.path.join(project_dir, ".pre-commit-config.yaml")