Add Github Actions workflow for building and publishing releases

[robertpanzer]

Thank you for opening a pull request and contributing to AsciidoctorJ!

Please take a bit of time giving some details about your pull request:

Kind of change

• Bug fix
• New non-breaking feature
• New breaking feature
• Documentation update
• Build improvement

Description

What is the goal of this pull request?

Build releases on Github actions.

How does it achieve that?

The PR has a new workflow for building releases.
I configured secrets for the signing key and the access token for Sonatype.
For now the idea for this to work is:

1. We update main so that it has a release version
2. We trigger the action manually on github.com or via CLI
3. GH runs a full build and publishes to a Sonatype staging repository and closes it

Going forward I'd like to switch to the way asciidoctor/asciidoctor does it by simply pushing a tag.

Are there any alternative ways to implement this?

Yes, for example build and publish release on tagging.

Are there any implications of this pull request? Anything a user must know?

Issue

If this PR fixes an open issue, please add a line of the form:

Fixes #Issue

Release notes

Please add a corresponding entry to the file CHANGELOG.adoc