docs: Added resource card to AdMob Ads page in DAC

The "Earn Revenue from AdMob Ads" page now includes a new related
resource, a link to the Udacity course teaching developers how to
display ads in an Android app using AdMob.

Link to staged page:
https://khufnagle-cloudstage-test.googleplex.com/distribute/monetize/ads.html#related_resources

Bug: 27275529
Change-Id: Ib1aa34ec5065c801672e3b8dd07b05b2b86cf783