Update breaking changes from starter-pack and improve workflow

- There are recent breaking changes in canonical/sphinx-docs-starter-pack
that causes our workflow to fail. Updated those changes in COU to align.
- Update the workflow to only trigger documentation checks if a PR
modifies `docs/`. This will prevent un-necessary runs. I'm keeping the
on.push as is because I think it is good to have a way to notify us
about dependency breakage (those issues are not affecting live doc site).

.github/workflows/automatic-doc-checks.yml

@@ -3,6 +3,8 @@ name: Main Documentation Checks
 on:
   - push
   - pull_request
+      paths:
+        - 'docs/**'
   - workflow_dispatch
 
 concurrency:

docs/.custom_wordlist.txt

@@ -20,4 +20,5 @@ OpenStackApplication
 yaml
 backoff
 BACKOFF
-websockets
+websockets
+enablement

docs/Makefile

@@ -77,10 +77,10 @@ serve: html
 clean: clean-doc
 	@test ! -e "$(VENVDIR)" -o -d "$(VENVDIR)" -a "$(abspath $(VENVDIR))" != "$(VENVDIR)"
 	rm -rf $(VENVDIR)
-	rm -rf .sphinx/.doctrees
 
 clean-doc:
 	git clean -fx "$(BUILDDIR)"
+	rm -rf .sphinx/.doctrees
 
 spelling: html
 	. $(VENV) ; python3 -m pyspelling -c .sphinx/spellingcheck.yaml
@@ -93,9 +93,9 @@ woke: woke-install
 	    -c https://github.com/canonical/Inclusive-naming/raw/main/config.yml
 
 pa11y: pa11y-install html
-	find $(BUILDDIR) -name *.html -exec $(PA11Y) {} \;
+	find $(BUILDDIR) -name *.html -print0 | xargs -n 1 -0 $(PA11Y)
 
 # Catch-all target: route all unknown targets to Sphinx using the new
 # "make mode" option.  $(O) is meant as a shortcut for $(SPHINXOPTS).
 %: Makefile
-	. $(VENV); $(SPHINXBUILD) -M $@ "$(SOURCEDIR)" "$(BUILDDIR)" $(SPHINXOPTS) $(O)
+	. $(VENV); $(SPHINXBUILD) -M $@ "$(SOURCEDIR)" "$(BUILDDIR)" $(SPHINXOPTS) $(O)