ci: doc-build: Reduce parallel job count to 14

This commit reduces the parallel job count for building the documentation
from the default value of 16 (i.e. `-j auto` aka. the vCPU count) to 14
because the total available RAM in the runners is 32GiB and each
sphinx-build process may use more than 2GiB of RAM, which can lead to
out-of-memory conditions.

Note that the HTML doc build process was parallelised prior to this change
in spite of the lack of `-j auto` in the CI workflow because the default
`SPHINXOPTS` includes `-j auto` and only `SPHINXOPTS_EXTRA` was specified
-- this commit explicitly adds the `SPHINXOPTS` for it with the default
value specified in doc/CMakeLists.txt to make the job count configurable
while ensuring the previous behaviour is preserved.

Signed-off-by: Stephanos Ioannidis <root@stephanos.io>

.github/workflows/doc-build.yml

@@ -18,6 +18,9 @@ env:
   # so we fetch that through pip.
   CMAKE_VERSION: 3.20.5
   DOXYGEN_VERSION: 1.9.6
+  # Job count is set to 2 less than the vCPU count of 16 because the total available RAM is 32GiB
+  # and each sphinx-build process may use more than 2GiB of RAM.
+  JOB_COUNT: 14
 
 jobs:
   doc-file-check:
@@ -132,7 +135,11 @@ jobs:
         else
           DOC_TARGET="html"
         fi
-        DOC_TAG=${DOC_TAG} SPHINXOPTS_EXTRA="-q -t publish" make -C doc ${DOC_TARGET}
+
+        DOC_TAG=${DOC_TAG} \
+        SPHINXOPTS="-j ${JOB_COUNT} -W --keep-going -T" \
+        SPHINXOPTS_EXTRA="-q -t publish" \
+        make -C doc ${DOC_TARGET}
 
         # API documentation coverage
         python3 -m coverxygen --xml-dir  doc/_build/html/doxygen/xml/ --src-dir include/ --output doc-coverage.info
@@ -245,7 +252,10 @@ jobs:
           DOC_TAG="development"
         fi
 
-        DOC_TAG=${DOC_TAG} SPHINXOPTS="-q -j auto" LATEXMKOPTS="-quiet -halt-on-error" make -C doc pdf
+        DOC_TAG=${DOC_TAG} \
+        SPHINXOPTS="-q -j ${JOB_COUNT}" \
+        LATEXMKOPTS="-quiet -halt-on-error" \
+        make -C doc pdf
 
     - name: upload-build
       if: always()