Add standalone documentation module with Antora

[ccremer]

Summary

• Adds Antora documentation module (opt-out)
• Adds Make targets for local preview
• Adds Make target also for easy deployment to GitHub Pages (opt-out file-wise, opt-in because first the gh-pages branch has to be created before GH Pages is active)

Note: The included playbook is for GH pages. The Antora module can also be built externally with existing VSHN's Antora playbooks.

Checklist

• Categorize the PR by setting a good title and adding one of the labels:
  bug, enhancement, documentation, change, breaking, dependency
  as they show up in the changelog
• Update tests.

[ccremer]

So far K8up is the only Go-based software repo where we included Antora documentation module AFAIK.
I added this Antora scaffold to this template to help prepare specific documentation for a software component.
For example, to document upcoming Odoo-specific adapter for APPUiO Cloud