Revert "docs: Rename Code of Conduct to fix GitHub detection"

This reverts commit 50d28de10b7209513c76161e65b66c42887be4ec.

Unfortunately, it seems GitHub requires the code of conduct to:
1. be named CODE_OF_CONDUCT
2. have a .md file extension

This commit restores external links to the CoC for now.

CONTRIBUTING.rst

@@ -71,4 +71,4 @@ Commit Guidelines
 Conduct Guidelines
 ------------------
 
-- Contributors to the OBS Project are expected to abide by the OBS Project Code of Conduct: https://github.com/obsproject/obs-studio/blob/master/CODE_OF_CONDUCT.rst
+- Contributors to the OBS Project are expected to abide by the OBS Project Code of Conduct: https://github.com/obsproject/obs-studio/blob/master/COC.rst