Explicitly specify sphinx-build cache directories

Explicitly specify sphinx-build cache directories

When sphinx-build is invoked without the -d command line switch, the
default path to the directory in which cached environment and doctree
files are placed is OUTPUTDIR/.doctrees.  This causes the contents of
such cache directories to needlessly be included in BIND release
directories.  Avoid that by employing the -d command line switch to make
each sphinx-build process use a cache directory outside the output
directory.  Make sure these cache directories are separate from each
other as well, to prevent multiple sphinx-build processes running in
parallel from interfering with each other.