docs: Makefile: Fix make cleandocs by deleting generated .rst files

The script tools/net/ynl/ynl-gen-rst.py (YNL_TOOL) generates several .rst
files (YNL_INDEX, YNL_RST_FILES) in Documentation/networking/netlink_spec
(YNL_RST_DIR) which are not deleted by make cleandocs.

Fix make cleandocs by deleting the generated .rst files.

Signed-off-by: Thorsten Blum <thorsten.blum@toblux.com>
Reviewed-by: Vegard Nossum <vegard.nossum@oracle.com>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
Link: https://lore.kernel.org/r/20240208145001.61769-1-thorsten.blum@toblux.com