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: 2ff97847 ("docs: document zink's gl > 3.0 requirements")
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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: 2ff97847 ("docs: document zink's gl > 3.0 requirements")