docs/cxl: Add initial Compute eXpress Link (CXL) documentation.

docs/cxl: Add initial Compute eXpress Link (CXL) documentation.

Provide an introduction to the main components of a CXL system,
with detailed explanation of memory interleaving, example command
lines and kernel configuration.

This was a challenging document to write due to the need to extract
only that subset of CXL information which is relevant to either
users of QEMU emulation of CXL or to those interested in the
implementation.  Much of CXL is concerned with specific elements of
the protocol, management of memory pooling etc which is simply
not relevant to what is currently planned for CXL emulation
in QEMU.  All comments welcome

Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Message-Id: <20220429144110.25167-43-Jonathan.Cameron@huawei.com>
Reviewed-by: Michael S. Tsirkin <mst@redhat.com>
Signed-off-by: Michael S. Tsirkin <mst@redhat.com>