Add make clean target (JuliaDocs#2696)

This deletes all files generated during local development workflows. Cf.

    git ls-files --others --ignored --exclude-standard --directory

Helpful to run before switching branches.

Author: goerz

Makefile

@@ -20,11 +20,31 @@ themes:
 test:
 	${JULIA} --project -e 'using Pkg; Pkg.test()'
 
+clean:
+	rm -f Manifest.toml
+	rm -f docs/Manifest.toml
+	rm -f docs/src/release-notes.md
+	rm -rf docs/dev
+	rm -rf docs/build
+	rm -rf test/docstring_links
+	rm -rf test/docstring_links/build
+	rm -rf test/docsxref/build
+	rm -rf test/doctests/builds
+	rm -rf test/errors/build
+	rm -rf test/examples/builds
+	rm -rf test/missingdocs/build
+	rm -rf test/nongit/build
+	rm -rf test/plugins/build
+	rm -rf test/quietly-logs
+	rm -rf test/workdir/builds
+
+
 help:
 	@echo "The following make commands are available:"
 	@echo " - make changelog: update all links in CHANGELOG.md's footer"
 	@echo " - make docs: build the documentation"
 	@echo " - make test: run the tests"
 	@echo " - make themes: compile Documenter's native CSS themes"
+	@echo " - make clean: remove generated files"
 
 .PHONY: default docs-instantiate themes help changelog docs test