gh-145455: Show output of blurb & sphinx-build version commands (GH-145457)

In gh-145455, an outdated dependency caused an import error that was not
printed out (`2>&1`); the message instead said that the tools are missing.

Don't redirect stderr, to show warnings and failures.

Also, switch `blurb` to output a version on a single line (`--version` rather
than `help`), and, and don't redirect stdout either.
This results in two version info lines being printed out. These get drowned
in typical Sphinx output, and can be helpful when debugging.
(cherry picked from commit f1de65b)

Co-authored-by: Petr Viktorin <encukou@gmail.com>

Author: encukou

Doc/Makefile

@@ -58,7 +58,7 @@ build:
 	@if [ -f  ../Misc/NEWS ] ; then \
 		echo "Using existing Misc/NEWS file"; \
 		cp ../Misc/NEWS build/NEWS; \
-	elif $(BLURB) help >/dev/null 2>&1 && $(SPHINXBUILD) --version >/dev/null 2>&1; then \
+	elif $(BLURB) --version && $(SPHINXBUILD) --version ; then \
 		if [ -d ../Misc/NEWS.d ]; then \
 			echo "Building NEWS from Misc/NEWS.d with blurb"; \
 			$(BLURB) merge -f build/NEWS; \