Docs: Turn warnings into errors

Use the -W (--fail-on-warning) flag of sphinx-build to tell it to exit
with an error if any warnings are generated. This is intended to fail
the coreboot-docs-gerrit build to help catch issues.

To allow all warnings to be output in the same build, use the
--keep-going flag so that the author is able to see all issues and
address them after a single build. Note that this behavior is enabled by
default as of Sphinx 8.1 and this option may be removed in the future.
It is added here for compatibility with older versions of Sphinx,
including the doc.coreboot.org container which uses 7.2.6.

Change-Id: I3aa564b79d4d4125a3800023b1b805bf4a50b10a
Signed-off-by: Nicholas Chin <nic.c3.14@gmail.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/84886
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Felix Singer <service+coreboot-gerrit@felixsinger.de>

Documentation/Makefile

@@ -5,7 +5,7 @@
 #
 
 BUILDDIR ?= _build
-SPHINXOPTS ?= -j auto
+SPHINXOPTS ?= -j auto -W --keep-going
 
 export SPHINXOPTS