Add backend and compat CODEOWNERS

Author: marvinpfoertner

.github/CODEOWNERS

@@ -7,6 +7,13 @@
 /docs/source/_ext/probnum-config-options.py @marvinpfoertner
 /docs/source/api/config.rst @marvinpfoertner
 
+# Compute Backends
+/src/probnum/backend @marvinpfoertner @JonathanWenger
+/tests/test_backend @marvinpfoertner @JonathanWenger
+
+# Compatibility Functions
+/src/probnum/compat @marvinpfoertner @JonathanWenger
+
 # Abstract Probabilistic Numerical Method
 /src/probnum/_pnmethod/ @JonathanWenger
 /tests/test_pnmethod/ @JonathanWenger