docs: introduce and use :ext:-role for GL/VK extensions

We refer to GL and VK extensions quite frequently, and we sometimes provide a link, and sometimes not. It would be nice if we were always linking, wouldn't it? This implements that, using a custom :ext:-role.

I also fixed a few cases where an extension was typoed, to make sure we produce valid links. I've tried to make sure I validated every produced link as well.