Fail docs if warnings

Author: domdfcoding

.github/workflows/docs_test_action.yml

@@ -34,4 +34,4 @@ jobs:
         with:
           pre-build-command: python -m pip install tox
           docs-folder: "doc-source/"
-          build-command: "tox -e docs -- "
+          build-command: "tox -e docs -- -W "

repo_helper.yml

@@ -16,6 +16,7 @@ use_whey: true
 min_coverage: 100
 sphinx_html_theme: furo
 preserve_custom_theme: true
+docs_fail_on_warning: true
 
 conda_channels:
  - conda-forge