SpringBoot Configure more of Asciidoctor by convention

We repeat this a few times now:

repositories {
    maven {
        url "https://repo.spring.io/release"
        mavenContent {
            includeGroup "io.spring.asciidoctor"
            includeGroup "io.spring.asciidoctor.backends"
        }
    }
}

Comment From: wilkinsona

There's a bit more that we could do with things like the asciidoctorExtensions configuration as well.