Detach Quarkus Platform version from Quarkus version in docs

Currently just the

quarkus_version

Antora attribute is used both for the Quarkus platform and the version of Quarkus itself. We need to split it as Quarkus Platform is a separate project with a separate versioning scheme.

Recently it happened that the com.redhat.quarkus.platform:quarkus-bom:2.13.5.Final-redhat-00002 referenced the quarkus-core:2.13.5.Final-redhat-00003, so the versions were not the same.

The result is that users wanting to generate a project use a non-existent quarkus-maven-plugin, thus are unable to generate a project.