docs/features: Track Panfrost

docs/features: Track Panfrost

Mark support for Panfrost with the PAN_MESA_DEBUG=gles3 flag set (which
exposes a few buggier features for GLES 3.0, but we're actually quite
close to conformance. I expect this to become default in a few weeks),
based on what's supported for Mali T860 (our flagship). Less features
are supported on Mali T720 due to h/w limitations, and Bifrost support
is very much still in the pipes but will support all this soon enough.

Closes: #255
Signed-off-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/5791>