Sphinx-generated documentation built in GitLab CI is sometimes defective

@pspacek noticed that the latest PDF version of the BIND 9 ARM is missing the entire "BIND 9 Configuration Reference" chapter:

https://ftp.isc.org/isc/bind9/9.17.9/doc/arm/Bv9ARM.pdf

The same problem seems to be affecting the PDF for 9.17.8:

https://ftp.isc.org/isc/bind9/9.17.8/doc/arm/Bv9ARM.pdf

but not for 9.17.7:

https://ftp.isc.org/isc/bind9/9.17.7/doc/arm/Bv9ARM.pdf

None of the 9.16.x PDFs seem to be broken this way.

I could not reproduce this problem locally and also confirmed there were no changes in the doc/ directory between 9.17.7 and 9.17.8 that could have caused this. Upon further investigation, I discovered that the problem is caused by using parallel make jobs (-j) when running make doc in GitLab CI - this causes multiple sphinx-build instances to run simultaneously in the same working directory, which means they can stomp on each other's data. Unfortunately, problems like this only triggered Sphinx warnings, not errors, so the relevant GitLab CI jobs have been silently passing.

This conclusion means that all forms of our documentation might have been malformed in one way or another ever since we migrated to Sphinx, i.e. since May 2020 (see !1761 (merged), !3536 (merged)). Looking at GitLab CI job traces from the past 2 months, it can be estimated that the problem has a roughly 15% chance of getting triggered for any Sphinx documentation build job. (See correction below.)