add docs on debug mode

docs/other/debug.md

@@ -0,0 +1,17 @@
+To debug the provisioning step Drupal VM can run the `ansible-playbook` command in verbose mode if you set the `DRUPALVM_DEBUG` environment variable.
+
+```sh
+# verbose mode: ansible-playbook -v
+DRUPALVM_DEBUG=1 vagrant provision
+DRUPALVM_DEBUG=v vagrant provision
+
+# for even more messages: ansible-playbook -vvv
+DRUPALVM_DEBUG=vvv vagrant provision
+
+# for debug mode: ansible-playbook -vvvv
+DRUPALVM_DEBUG=vvvv vagrant provision
+```
+
+In addition to running in verbose mode this also enables deprecation warnings.
+
+For debugging information related to Vagrant see [documentation](https://www.vagrantup.com/docs/other/debugging.html).

mkdocs.yml

@@ -63,6 +63,7 @@ pages:
     - 'Passing on CLI arguments to ansible': 'extending/ansible-args.md'
     - 'Pre- and Post-Provision Scripts': 'extending/scripts.md'
   - Other Information:
+    - 'Debug mode': 'other/debug.md'
     - 'Networking Notes': 'other/networking.md'
     - 'Vagrant LXC provider': 'other/vagrant-lxc.md'
     - 'Improving Performance': 'other/performance.md'