docs: use alabaster theme

It's not all perfect (yet) but way cleaner and simpler to use than
the sphinx one.

Custom do the scrolling for the fixed side bar and make some other
slight adjustments.

Main issue for now is that the "Developer Appendix" is always shown
in the navigation tree, but we only include that toctree for
devbuilds...

Signed-off-by: Thomas Lamprecht <t.lamprecht@proxmox.com>

docs/Makefile

@@ -76,6 +76,8 @@ onlinehelpinfo:
 .PHONY: html
 html: ${GENERATED_SYNOPSIS}
 	$(SPHINXBUILD) -b html $(ALLSPHINXOPTS) $(BUILDDIR)/html
+	cp images/proxmox-logo.svg $(BUILDDIR)/html/_static/
+	cp custom.css $(BUILDDIR)/html/_static/
 	@echo
 	@echo "Build finished. The HTML pages are in $(BUILDDIR)/html."