docs: Use HTTP_PORT by default

8000 is valuable real estate

Author: tony

docs/Makefile

@@ -177,4 +177,4 @@ dev:
 	$(MAKE) -j watch serve
 
 start:
-	poetry run sphinx-autobuild "$(SOURCEDIR)" "$(BUILDDIR)" $(SPHINXOPTS) $(O)
+	poetry run sphinx-autobuild "$(SOURCEDIR)" "$(BUILDDIR)" $(SPHINXOPTS) --port ${HTTP_PORT} $(O)