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

Documentation/Makefile

@@ -176,6 +176,7 @@ refcheckdocs:
 	$(Q)cd $(srctree);scripts/documentation-file-ref-check
 
 cleandocs:
+	$(Q)rm -f $(YNL_INDEX) $(YNL_RST_FILES)
 	$(Q)rm -rf $(BUILDDIR)
 	$(Q)$(MAKE) BUILDDIR=$(abspath $(BUILDDIR)) $(build)=Documentation/userspace-api/media clean