Revert "DOP-3265: Update illustrations and icons (#98)" (#99)

This reverts commit 3a20723.

Author: mmeigs

source/images/homepage-hero.png

@@ -28,7 +28,7 @@ Welcome to the MongoDB Documentation
    Find the guides, samples, and references you need to use the database,
    visualize data, and build applications on the MongoDB data platform.
 
-.. image:: /images/homepage-hero.svg
+.. image:: /images/homepage-hero.png
    :class: hero-img
    :alt: Homepage illustration
 
@@ -126,8 +126,7 @@ Community.
       :headline: Free Online Courses on MongoDB University
       :cta: Get started on MongoDB University
       :url: http://university.mongodb.com/?tck=docs_landing
-      :icon: /images/icons/university.svg
-      :icon-alt: MongoDB University icon
+      :tag: University
 
       Build your MongoDB expertise for free with in-depth lessons and
       videos on University.
@@ -136,8 +135,7 @@ Community.
       :headline: Collaborative Community Forums
       :cta: Join our Community
       :url: https://community.mongodb.com/?tck=docs_landing
-      :icon: /images/icons/ask-the-experts.svg
-      :icon-alt: MongoDB Ask The Experts icon
+      :tag: Community
 
       Learn and build relationships with like-minded developers using
       MongoDB.