doc: look for snippet titles/docs in output-dir rather than origdir
The snippet titles are XX/lily-XXXXXX.texidoc[LL]. These are created in Documentation/out-www/LL/ rather than ${build}/Documentation/
It seems this is currently masked, because the files are still found due to the plethora of -I flags.
Tested: diff -urN on out-www/webdoc shows no HTML differences