docs: add ASmap process via extraSources

[jurraca]

This is one of the few options where we need to copy a file over, so thought it would be worth a mention in this section of the docs.

Follow up might be an explicit option in the module, but not sure how to amend the extraSources based on a module config, if possible.

[0xB10C]

fwiw: For my personal use, I'm packaging asmap_latest.dat as package (which only contains the asmap_latest.dat binary). See https://github.com/0xB10C/nix/blob/master/pkgs/asmap-data/default.nix. I guess that could be an alternative to copying the file.

[jurraca]

ah yea, that's much nicer. Might make sense to have that package included in this repo? I don't think it make sense in kartograf.