add cim2 external resources to zotero

[lisa-wm]

in the cim2 README, there is a collection of extra material (https://github.com/slds-lmu/lecture_cim2).
everything related to still-relevant topic should be moved to our public zotero SLDS library (subcollection teaching_resources). please make good use of the zotero tags so this collection does not get out of hands.