Migrate documentation to Antora

[rwinch]

Migrate to Antora based documentaiton

[fmbenhassine]

Thank you for opening the PR! I tested it locally and seems to correctly generate the HTML version of the docs. However, a couple notes:

• Is it intended that resources are duplicated in modules/ROOT/assets/images and src/main/asciidoc/images?
• I noticed that the asciidoctor-maven-plugin was removed. Therefore the pdf and epub3 versions are not generated anymore. Is it possible to generate them with the antora-maven-plugin?
• The maven-deploy-plugin was removed as well. This was required to skip deploying the docs as part of the release process as we have a separate action for that (and that is on purpose). Anything needed to update here?

The goal is to be able to keep the distribution generation process as described in the REAME. If we have that, it won't be necessary to modify the release process (which I would prefer). Otherwise I am open to update the instructions if needed as well.

[rwinch]

Is it intended that resources are duplicated in modules/ROOT/assets/images and src/main/asciidoc/images?

Thanks for noticing this. It is not intentional and I've added a commit that corrects this.

I noticed that the asciidoctor-maven-plugin was removed. Therefore the pdf and epub3 versions are not generated anymore. Is it possible to generate them with the antora-maven-plugin?

At the moment we are not trying to do any other formats. We will likely investigate using Antora Assembler to output PDFs in a later iteration.

The maven-deploy-plugin was removed as well. This was required to skip deploying the docs as part of the release process as we have a separate action for that (and that is on purpose). Anything needed to update here?

The PR has a GitHub action that takes care of pushing your documentation for you. The build is structured in a way that rsync is preferred because it can copy only the files that have changed and sync a single version if that is all that is needed to be built. See https://github.com/spring-projects/spring-batch/pull/4423/files#diff-87e860af4b9db6c503ed680b6edb6333c6f8d7659f7d1a779a2487466bbc50f6R41

[fmbenhassine]

Rebased, squashed and merged as 2e8d506. Thank you for doing this!