docs: Add placeholder page for CI & testing information

Change-Id: Iebcac0827c6f715c6b804223cdcf2cc2e425120b

doc/source/contributor/ci.rst

+=====================================
+Continuous Integration (CI) & Testing
+=====================================
+
+This page is a placeholder for information on the Kolla Ansible Continuous
+Integration (CI) & testing setup.

doc/source/contributor/index.rst

@@ -22,4 +22,5 @@ We welcome everyone to join our project!
    bug-triage
    ptl-guide
    release-management
+   ci
    setup-identity-provider