The use of ATX headers (aka "hash syntax" is part of our markdown
standards as defined here:
https://docs.civicrm.org/dev/en/latest/markdownrules/#standards

To make this change, I did the following:
perl -0777 -i -pe 's|^(.*)\n-{3,}$|## $1|gm' `find . -name '*.md'`
perl -0777 -i -pe 's|^(.*)\n={3,}$|# $1|gm' `find . -name '*.md'`

I also tested the built HTML files before/after using git to make sure
that nothing actually changed in the build as a result of this commit