docs: add a minimal docs page for radv
RADV is the last driver in Mesa that doesn't have a webpage somewhere that we can link to from mesa3d.org. So let's make a minimal docs article for it where we link to relevant information elsewhere.
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RADV is the last driver in Mesa that doesn't have a webpage somewhere that we can link to from mesa3d.org. So let's make a minimal docs article for it where we link to relevant information elsewhere.