Docsite sphinx rm buildsite (and speed up docs build) (#20237)

Replace docs build-site.py with default-ish sphinx build

This seems to speed up docsite build _alot_. 

The Makefile.sphinx is the sphinx-quickstart generated makefile with a few changes.

The CPUS env var or 'nproc' output is used for the number of cpus passed to 'sphinx-build -j'

docs/docsite/Makefile

@@ -1,12 +1,11 @@
 SITELIB = $(shell python -c "from distutils.sysconfig import get_python_lib; print get_python_lib()")
 FORMATTER=../../hacking/module_formatter.py
 DUMPER=../../hacking/dump_playbook_attributes.py
-CPUS ?= 1
+CPUS := $(shell nproc)
 
 all: clean docs
 
-docs: clean directives modules staticmin
-	./build-site.py -j $(CPUS)
+docs: clean directives modules htmldocs
 	-(cp *.ico htmlout/)
 	-(cp *.jpg htmlout/)
 	-(cp *.png htmlout/)
@@ -15,16 +14,14 @@ variables:
 	(mkdir -p htmlout/)
 	dot variables.dot -Tpng -o htmlout/variables.png
 
-viewdocs: clean staticmin
-	./build-site.py -j $(CPUS) view
-
-htmldocs: staticmin
-	./build-site.py -j $(CPUS) rst
+htmldocs: directives modules staticmin
+	CPUS=$(CPUS) $(MAKE) -f Makefile.sphinx html
 
 webdocs: htmldocs
 
 clean:
 	-rm -rf htmlout
+	-rm -rf _build
 	-rm -f .buildinfo
 	-rm -f *.inv
 	-rm -rf *.doctrees