tools/makefile: add clean target (clean files generated by automated tasks)

Makefile

@@ -24,6 +24,10 @@ url_check: install
 	source .venv/bin/activate && \
 	hecat --config .hecat/url-check.yml
 
+.PHONY: clean # clean files generated by automated tasks
+clean:
+	rm -rf .venv/ licenses.yml platforms/ software/ tags/
+
 .PHONY: help # generate list of targets with descriptions
 help:
 	@grep '^.PHONY: .* #' Makefile | sed 's/\.PHONY: \(.*\) # \(.*\)/\1	\2/' | expand -t20