docs: Move mypy documentation from contributing to testing.

The testing section is more appropriate, since it's fundamentally part
of our CI system.

While we're at it, fix the fact that we were linking to GitHub, not
ReadTheDocs, in the run-mypy output.

tools/run-mypy

@@ -91,6 +91,6 @@ else:
 
 if rc != 0:
     print("")
-    print("See https://github.com/zulip/zulip/blob/master/docs/contributing/mypy.md for debugging tips.")
+    print("See https://zulip.readthedocs.io/en/latest/testing/mypy.html for debugging tips.")
 
 sys.exit(rc)