cxl: Fix cross-reference in documentation and add deprecation warning

commit 5731d41a ("cxl: Deprecate driver") labelled the cxl driver as
deprecated and moved the ABI documentation to the obsolete/ subdirectory,
but didn't update cxl.rst, causing a warning once ff7ff6eb ("docs:
media: Allow creating cross-references for RC ABI") was merged.

Fix the cross-reference, and also add a deprecation warning.

Fixes: 5731d41a ("cxl: Deprecate driver")
Reported-by: Bagas Sanjaya <bagasdotme@gmail.com>
Signed-off-by: Andrew Donnellan <ajd@linux.ibm.com>
Acked-by: Bagas Sanjaya <bagasdotme@gmail.com>
Signed-off-by: Madhavan Srinivasan <maddy@linux.ibm.com>
Link: https://patch.msgid.link/20250219064807.175107-1-ajd@linux.ibm.com