Build Antora only when publishing

[pderop]

This PR is an attempt to accelerate the build when using JDK17, and when you don't need to generate antora docs.

typically, it should accelerate commands like ./gradlew clean build

to build the documentation:

• ./gradlew antora (docs is generated to docs/build/site/ as before)
• or ./gradlew publishToMavenLocal

modifications:

• the docs/build.gradle is not making the build depending on antora anymore
• removed the following unnecessary declaration in reactor-netty/build.gradle, which comes from spring-pulsar, we don't need this, and it forces the docsZip to be created when building or assembling the project:

artifacts {
	archives docsZip
}

Fixes #3157