cxl: docs/linux - add cxl-driver theory of operation

Add docs for the CXL driver that explains the base devices,
decoder types, region types, mailbox interfaces, and decoder
programming.

Signed-off-by: Gregory Price <gourry@gourry.net>
Link: https://patch.msgid.link/20250512162134.3596150-10-gourry@gourry.net
Signed-off-by: Dave Jiang <dave.jiang@intel.com>

Author: Gregory Price

Documentation/driver-api/cxl/index.rst

@@ -35,6 +35,7 @@ that have impacts on each other.  The docs here break up configurations steps.
 
    linux/overview
    linux/early-boot
+   linux/cxl-driver
    linux/access-coordinates