docs: Add sphinx autobuild for auto rebuilding the docs in a local server

Add this tooling for easier docs development.

Author: bjoernricks

docs/Makefile

@@ -11,7 +11,10 @@ BUILDDIR      = build
 help:
 	@$(SPHINXBUILD) -M help "$(SOURCEDIR)" "$(BUILDDIR)" $(SPHINXOPTS) $(O)
 
-.PHONY: help Makefile
+.PHONY: help Makefile livehtml
+
+livehtml:
+	sphinx-autobuild "$(SOURCEDIR)" "$(BUILDDIR)" $(SPHINXOPTS) $(O)
 
 # Catch-all target: route all unknown targets to Sphinx using the new
 # "make mode" option.  $(O) is meant as a shortcut for $(SPHINXOPTS).