docs: add link to extension spec

We already do this for all the other VK extensions, so we might as well
do this for VK_EXT_vertex_attribute_divisor as well.

Fixes: 2ff97847d19 ("docs: document zink's gl > 3.0 requirements")
Reviewed-By: Mike Blumenkrantz <michael.blumenkrantz@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/7312>
1 file changed