Build: Trigger generator pipeline when source content has changed

[philipwindecker]

Background

This issue targets the build pipeline.

Current situation

The current setup for Antora builds in regular intervals.
However, this is both too often and too infrequent.
Builds are only required when changes are pushed.

Solution

Add repository_dispatch event and required token to all content repositories and the generator repo.
Optional: Create adaptive builds for PRs where the content in Antora is changed to that PR but do not publish these pages.

Additional fix

The original pipeline currently still works with Ubuntu 18.04.
Support for this version will be removed at the end of 2022.
Therefore, a switch to Ubuntu 22.04 has been integrated as well.