main/linux-lts: add -doc subpackage

In !35272 (merged) I add a linux-docs package, but I feel like this should rather be provided by the linux-lts aport. I also think it would be nice to have such documentation packaged in general.

Here's an example of what it looks like in Debian https://packages.debian.org/bullseye/all/linux-doc-5.10/filelist