We need an entry point guide for Messaging

[gsmet]

Apparently, the guide in the SmallRye Reactive Messaging metadata was pointing to https://quarkus.io/guides/reactive-messaging which redirects to the Reactive Event Bus page and has nothing to do with our Messaging stack.

For now I dropped the guide from the descriptor as we need a working page but I think we need a https://quarkus.io/guides/messaging entry point with links to the various guides we have for messaging.

When created, we need to update the extension descriptor for the SmallRye Reactive Messaging extension.

@cescoffier does it make sense?

[cescoffier]

Agreed, makes sense.

[quarkus-bot]

/cc @MichalMaler (documentation), @ebullient (documentation), @inoxx03 (documentation), @michelle-purcell (documentation), @ozangunalp (reactive-messaging), @rolfedh (documentation), @sheilamjones (documentation), @sunayna15 (documentation)