Add playbookProvider parameter to retrieve template playbook file from git repository
In addition to the full site build, authors/developers will often want to build the docs from a specific branch directly from that branch. In fact, this is one of the key reasons for using the Maven plugin to drive the Antora build. However, since Antora requires a playbook file to run, this leads to a proliferation of playbook files that have to be maintained (one per branch). To avoid this redundancy, the Antora plugin should be able to retrieve a template playbook from a location in a designed branch of a git repository (often the playbook repository/branch). This way, Antora's configuration can be managed centrally while still allowing authors/developers to run Antora within a branch.
Here's how this configuration might look:
<configuration>
<playbookProvider>
<repository>org-name/project-name</repository>
<branch>docs-build</branch>
<path>lib/templates/branch-antora-playbook.yml</path>
<checkLocalBranch>true</checkLocalBranch>
</playbookProvider>
</configuration>
If checkLocalBranch is set, the plugin will attempt to copy the file from a local branch of the current repository (making the assumption that the current repository matches the specified repository). If the repository is not set, this is the default behavior.
If a # PACKAGES
magic comment defined at the top of the template file, those packages should be set in the plugin configuration.