docs: replace the old User Mode Linux HowTo with a new one

The new HowTo migrates the portions of the old howto which
are still relevant to a new document, updates them to linux 5.x
and adds documentation for vector transports and other new
features.

Signed-off-by: Anton Ivanov <anton.ivanov@cambridgegreys.com>
Link: https://lore.kernel.org/r/20200917103557.26063-1-anton.ivanov@cambridgegreys.com
Signed-off-by: Jonathan Corbet <corbet@lwn.net>