docs: Redirect top-level make targets to Documentation/

To avoid redundancy about how to call into `Makefile.sphinx`,
only do that from the `Documentation/Makefile` and call into
that from the top level.

Change-Id: I99c462cdaf83d711e4b7c07b713d304274db8cb4
Signed-off-by: Nico Huber <nico.h@gmx.de>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/77406
Reviewed-by: Paul Menzel <paulepanter@mailbox.org>
Reviewed-by: Felix Singer <service+coreboot-gerrit@felixsinger.de>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Simon Glass <sjg@chromium.org>
diff --git a/Makefile b/Makefile
index fac1dca..5bc07e8 100644
--- a/Makefile
+++ b/Makefile
@@ -468,10 +468,10 @@
 	cscope -bR
 
 sphinx:
-	$(MAKE) -C Documentation -f Makefile.sphinx html
+	$(MAKE) -C Documentation sphinx
 
 sphinx-lint:
-	$(MAKE) SPHINXOPTS=-W -C Documentation -f Makefile.sphinx html
+	$(MAKE) SPHINXOPTS=-W -C Documentation sphinx
 
 symlink:
 	@echo "Creating Symbolic Links.."; \