docs: update gallium doxygen docs
Gallium's background as a Tungstend Graphics technology is no longer significant; it's a historical detail. Besides, since Tungsten Graphics were acquired by VMware more than a decade ago, the website no longer exists.
While we're at it, replace the docs link with a link to the mesa docs, and point to archive.org copy of the Tungsten Graphics paper.
Closes: #2770 (closed)