docs: Add make rule to prepare venv similar to RTD build env

docs/Makefile

@@ -17,9 +17,19 @@ help:
 livehtml:
 	@$(SPHINXAUTOBUILD) --watch ../leapp "$(SOURCEDIR)" "$(BUILDDIR)/html" $(SPHINXOPTS) $(0)
 
+# Prepare virtualenv similar to the environment used during build on Read The Docs and install required packages
+env:
+	PY_VER=$$(sed -n -e 's/^\s*python:\s"\(.*\)"/\1/p' ../.readthedocs.yaml); \
+	echo $$PY_VER; \
+	rm -fr ./venv; \
+	virtualenv venv --python=python$$PY_VER; \
+	source venv/bin/activate; \
+	# pip install -r doesn't really work with relative paths :), so keeping it in the project root
+	pushd .. && pip install -r requirements-docs.txt; popd
+
 # Catch-all target: route all unknown targets to Sphinx using the new
 # "make mode" option.  $(O) is meant as a shortcut for $(SPHINXOPTS).
 %: Makefile
 	@$(SPHINXBUILD) -M $@ "$(SOURCEDIR)" "$(BUILDDIR)" $(SPHINXOPTS) $(O)
 
-.PHONY: all help Makefile
+.PHONY: all help Makefile env