docs: add how to get neutron ML2 mech calls

Added some information on how you can debug and see what inputs the ML2
mech is called with.

docs/deploy-guide/debug-neutron.md

@@ -0,0 +1,35 @@
+# Debugging Neutron
+
+## Debugging ML2
+
+To debug ML2 and see the messages that are flowing into the ML2
+mechanism you can add the following snippet into your `neutron.yaml`
+for your environment.
+
+```yaml
+conf:
+  plugins:
+    ml2_conf:
+      ml2:
+        # this line just aims to add 'logger' but its
+        # replacing so you'll need to pay attention
+        # to any changes your environment might have
+        # from the default
+        mechanism_drivers: "logger,understack,ovn"
+  logging:
+    loggers:
+      # for 'keys' we are attempt to append 'mechanism_logger'
+      # but the way YAML are merged you will need to include
+      # all other items in the list here as well.
+      keys:
+        - ...
+        - mechanism_logger
+    logger_mechanism_logger:
+      level: DEBUG
+      handlers: stdout
+      qualname: mechanism_logger
+```
+
+Once you deploy this the `neutron-server` pod will now log everything that the
+ML2 drivers receive. The log line will have `called with network settings` in it.
+That message will be prefixed with the ML2 method name like `create_network_postcommit`.

mkdocs.yml

@@ -124,6 +124,8 @@ nav:
     - deploy-guide/extra-regions.md
     - deploy-guide/external-argocd.md
     - deploy-guide/add-remove-app.md
+    - Debugging:
+      - deploy-guide/debug-neutron.md
   - 'User Guide':
     - user-guide/index.md
     - user-guide/openstack-cli.md