chore: update CONTRIBUTING doc (#1639)

This PR updates [CONTRIBUTING.rst](https://github.com/googleapis/google-api-python-client/blob/main/CONTRIBUTING.rst) to use the templated version from synthtool templates [here](https://github.com/googleapis/synthtool/blob/master/synthtool/gcp/templates/python_library/CONTRIBUTING.rst).

Fixes #1634 🦕
diff --git a/owlbot.py b/owlbot.py
index 0d995c1..4a03f4b 100644
--- a/owlbot.py
+++ b/owlbot.py
@@ -35,6 +35,9 @@
 # Move scripts folder needed for samples CI
 s.move(templated_files / 'scripts')
 
+# Copy CONTRIBUTING.rst
+s.move(templated_files / 'CONTRIBUTING.rst')
+
 # ----------------------------------------------------------------------------
 # Samples templates
 # ----------------------------------------------------------------------------