docs/panfrost: Document drm-shim usage

Document how to build and use Panfrost's drm-shim implementation. I hope by
documenting this process, other Mesa developers are better able to test
Panfrost. In particular, this allows developers without Mali hardware to run
shader-db for any Mali target, which may be useful for debugging regressions
from common NIR changes.

drm-shim is not a substitute for testing against real hardware.

Special thanks to Emma Anholt and Icecream95 for building this infrastructure.

Signed-off-by: Alyssa Rosenzweig <alyssa@collabora.com>
Part-of: <!15930>
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