add Makefile target for single HTML page (#25313)

docs/docsite/Makefile

@@ -8,6 +8,11 @@ else
 CPUS ?= $(shell nproc)
 endif
 
+assertrst:
+ifndef rst
+	$(error specify document or pattern with rst=somefile.rst)
+endif
+
 all: docs
 
 docs: clean htmldocs
@@ -51,3 +56,7 @@ staticmin:
 
 epub:
 	(CPUS=$(CPUS) make -f Makefile.sphinx epub)
+
+htmlsingle: assertrst
+	sphinx-build -j $(CPUS) -b html -d _build/doctrees ./rst _build/html rst/$(rst)
+	@echo "Output is in _build/html/$(rst)"