Add dissection sources to items' "Obtaining" section

[Terrorforge]

Monster entries show what you can get by dissecting them, but items don't show which monsters they can be dissected out of. E.g. gigantic naked mole-rat shows that dissecting it yields a troglobite sample, but the troglobite sample page doesn't tell you it can be acquired by dissecting a gigantic naked mole-rat.

It would be convenient if the item listed dissection sources the same ways they already do list butcher sources.

[nornagon]

done! also fixed an issue where mutagen_group and other harvest groups weren't properly understood by the guide.