Ignore .idea files (#84)

# Ignore .idea files

## ♻️ Current situation & Problem
Currently, `.idea` files of JetBrains editors are versioned. 


### Code of Conduct & Contributing Guidelines 

By submitting creating this pull request, you agree to follow our [Code
of
Conduct](https://github.com/StanfordBDHG/.github/blob/main/CODE_OF_CONDUCT.md)
and [Contributing
Guidelines](https://github.com/StanfordBDHG/.github/blob/main/CONTRIBUTING.md):
- [x] I agree to follow the [Code of
Conduct](https://github.com/StanfordBDHG/.github/blob/main/CODE_OF_CONDUCT.md)
and [Contributing
Guidelines](https://github.com/StanfordBDHG/.github/blob/main/CONTRIBUTING.md).

Co-authored-by: Paul Schmiedmayer <PSchmiedmayer@users.noreply.github.com>

.gitignore

@@ -8,3 +8,6 @@
 
 .DS_Store
 .env
+
+# editors
+.idea/