graphics/openexr-website-docs: unbreak and update to 3.3.2

And while here, decouple our PORTVERSION from the openexr version so we
don't break builds.

v3.3.2 seems to have a regression and causes a warning around Latest
News inserts on one site, so add -DSPHINX_OPTS= to override the default
of -W which means "treat warnings as errors", as of 3.3.2, I see one
warning.