drm/i915: Document the Virtual Engine uAPI

A little bit of documentation covering the topics of engine discovery,
context engine maps and virtual engines. It is not very detailed but
supposed to be a starting point of giving a brief high level overview of
general principles and intended use cases.

v2:
 * Have the text in uapi header and link from there.

v4:
 * Link from driver-uapi.rst.

Signed-off-by: Tvrtko Ursulin <tvrtko.ursulin@intel.com>
Cc: Daniel Vetter <daniel@ffwll.ch>
Acked-by: Daniel Vetter <daniel.vetter@ffwll.ch>
Link: https://patchwork.freedesktop.org/patch/msgid/20210618150036.2507653-1-tvrtko.ursulin@linux.intel.com