Update Promos on the hompage

The Hero has been updated to the RHOAR launch so we need to look at updating the promos.

We moved out the Adv. EAP cheatsheet from the hero so it is available as a promo.

The global list of heros/promos:
https://docs.google.com/spreadsheets/d/1_d6obHWsbgokPkYReearHa7QeuChMwL6xqvRRuXf6_o/edit#gid=1594789456

Current listing:
1. .Net Core 2.0 now available
2. Migrating to Microservics databases ebook
3. Explore .io
4. Building Reactive Microservices in Java ebook
5: Adv. Linux Cheat Sheet.

Available unused promos to use as replacements:
Promo for Intro to EAP Cheat Sheet (https://issues.jboss.org/browse/DESIGN-996)
Promo for "Advanced EAP Cheat Sheet" (https://issues.jboss.org/browse/DESIGN-999)
Promo for Mobile Cheat Sheet (https://issues.jboss.org/browse/DESIGN-992)