style-guide: frontend: describe directory naming style (#38607)

* style-guide: frontend: describe directory naming style

* apply recommended changes

Co-authored-by: Ursula Kallio <73951760+osg-grafana@users.noreply.github.com>

Co-authored-by: Ursula Kallio <73951760+osg-grafana@users.noreply.github.com>

contribute/style-guides/frontend.md

@@ -244,6 +244,8 @@ For files exporting multiple utility functions, use the name that describes the
 - Use `reducers.ts` Redux reducers.
 - Use `*.test.ts(x)` for test files.
 
+- Use kebab case for directory names: lowercase, words delimited by hyphen ( `-` ). For example, `features/new-important-feature/utils.ts`.
+
 ### Code organization
 
 Organize your code in a directory that encloses feature code: