chore: rename CONTRIBUTIONS.md to CONTRIBUTING.md

- Rename packages/web/CONTRIBUTIONS.md to CONTRIBUTING.md to follow standard open source naming convention
- Update reference in packages/web/README.md to point to the renamed file

packages/web/README.md

@@ -151,6 +151,6 @@ requests:
     Meshtastic nodes.
 
 Please review our
-[Contribution Guidelines](https://github.com/meshtastic/web/blob/main/packages/web/CONTRIBUTIONS.md)
+[Contribution Guidelines](https://github.com/meshtastic/web/blob/main/packages/web/CONTRIBUTING.md)
 before submitting a pull request. We appreciate your help in making the project
 better!