add(docs): PORT option for live-reload

Helps avoiding conflicts when previewing other versions of the docs, or
using other software running a local web server.

docs/Makefile

@@ -8,6 +8,8 @@ SPHINXBUILD   ?= sphinx-build
 SOURCEDIR     = source
 BUILDDIR      = build
 
+PORT ?= 8000
+
 # Put it first so that "make" without argument is like "make help".
 help:
 	@$(SPHINXBUILD) -M help "$(SOURCEDIR)" "$(BUILDDIR)" $(SPHINXOPTS) $(O)
@@ -34,6 +36,7 @@ $(VERSIONS):
 
 livehtml:
 	sphinx-autobuild "$(SOURCEDIR)" "$(BUILDDIR)/livehtml" $(SPHINXOPTS) $(O) \
+		--port "$(PORT)" \
 		--watch ../crypto_condor \
 		--watch ../tests \
 		--ignore *.dat