Use separate sphinx-build cache directories

Use separate sphinx-build cache directories

Simultaneously starting multiple sphinx-build instances with the -d
command line switch set to a common value (which is what happens when
e.g. "make -j6 doc" is run) causes intermittent problems which we failed
to notice before because they only trigger Sphinx warnings, not errors,
e.g.:

    WARNING: toctree contains ref to nonexisting file 'reference'

The message above is not triggered because doc/arm/reference.rst is
actually missing from disk at any point, but rather because a temporary
file created by one sphinx-build instance gets truncated by another one
working in parallel (the confusing message quoted above is logged
because of an overly broad "except" statement in Sphinx code).

Prevent this problem from being triggered by making each sphinx-build
process use its own dedicated cache directory.