Add landing pages for Spitzer, 2MASS, and COSMOS

#206 added a landing page for Euclid and #220 is adding them for SPHEREx, WISE, simulated data, and special topics. Should we add them for Spitzer, 2MASS, and COSMOS as well? Each of those only has one notebook, so there's less need for a landing page. We may still want them for equity and/or consistency.