Doc Vert.x 4.0.2 OpenJ9 related Power images (0.21.0) release [MULTIARCH]

Update docs with information about support for Power.

Update Docs :

• If there is anything that mentions OpenJDK tuning config etc, we will have to edit because we'll be using OpenJ9 and that will be different tuning.
• If we mention anything about the image name it might need to be changed
• There may be some unsupported things we'll have to note in our docs if they're using OpenShift on Power, but there isn't a list of these things yet.
• If the documentation mentions supported configs (not all of them have it in the docs) there will be updates there
• To include the necessary release note(s)

Here is the issue tracking work from when we added Z support (this task should be similar).

Details of what we updated for Z

• Vert.x https://docs.google.com/document/d/1vsYZosCpW_ohgSmTddg4pReDOtawfJ26JEeqYfGW9lQ/edit#

In CNR the runtimes share many of the same content files for reuse in all the guides.

cc: rahusing