More changes to improve (i.e., make less obfuscated/more modular) the
build system for the documentation in preparation for implementing XML
Tested by Alan W. Irwin <email@example.com> using
scripts/generate_website.sh (which includes using the -DBUILD_DOC=ON
cmake option and the prebuild_dist www-install-base, and www-install
targets, but which does not test the -DPREBUILT_DOC=ON case).