Update documentation: require Antora 3.2.0-alpha.2
The README previously still stated that Antora 3.1 could be used. But the "import" feature runs during componentsRegistered
event. This event is only available as of Antora 3.2.0-alpha.2.
Mention it in the Changelog too, to highlight this change in requirement.
Notes
I use the same syntax and content as in https://gitlab.com/opendevise/oss/antora-release-line-extension.