fix: markdown linting path and handles no files found

fix: pin markdownlnt version to 0.9.35

fix the issue caused by Markdown linter updates
canonical/sphinx-docs-starter-pack#509

fix: do not fail if no markdown files found

Author: tang-mm

docs/Makefile

@@ -74,7 +74,7 @@ pa11y-install:
 		}
 
 pymarkdownlnt-install:
-	@. $(VENV); test -d $(SPHINXDIR)/venv/lib/python*/site-packages/pymarkdown || pip install pymarkdownlnt
+	@. $(VENV); test -d $(VENVDIR)/lib/python*/site-packages/pymarkdown || pip install pymarkdownlnt==0.9.35
 
 install: $(VENVDIR)
 
@@ -110,7 +110,16 @@ pa11y: pa11y-install html
 	find $(BUILDDIR) -name *.html -print0 | xargs -n 1 -0 $(PA11Y)
 
 lint-md: pymarkdownlnt-install
-	@. $(VENV); pymarkdownlnt --config $(SPHINXDIR)/.pymarkdown.json scan --recurse --exclude=./$(SPHINXDIR)/** $(SOURCEDIR)
+	@. $(VENV); pymarkdownlnt --config $(SPHINXDIR)/.pymarkdown.json \
+	  --return-code-scheme explicit \
+	  scan --recurse \
+	  --exclude=$(SPHINXDIR)/** $(SOURCEDIR); \
+	status=$$?; \
+	if [ $$status -eq 1 ]; then \
+		echo "pymarkdownlnt: no Markdown files found (NO_FILES_TO_SCAN)."; \
+		exit 0; \
+	fi; \
+	exit $$status
 
 vale-install: install
 	@. $(VENV); test -f $(VALE_CONFIG) || python3 $(SPHINXDIR)/get_vale_conf.py