Branch specific variables (attributes)

[mmattel]

After a discussion with @xoxys the issue raised, that we need variables (attributes in terms of atntoa/asciidoc) per branch as content in branches needs to be specific limited to branch data.

The good news is, that there is a solution for this:

1. All what we want to be availabe in all mdoules and branches can be placed in site.yml as we do right now. This is or can be a good thing for global variables that do not change per module or branch. Each variable needs to be checked, for the existing and upcoming processes if this is a good approach.
2. Variables (attributes) we only want to have for a particular branch or where the value changes per branch and must not be the same for other branches must go to antora.yml

To do so, we need to adopt in all doc repos and in all branches two files:

3. site.yml
4. antora.yml

and for the creation of the pdf´s in all doc repos

5. I need to adopt the makepdf script because I currently only read site.yml, which must be extended to antora.yml

Finally, we need to update the new branch documentation to reflect the changes made.

The switch must be tested first and be handled carefully and will most likely NOT be done in a single step.

For details see:
https://docs.antora.org/antora/2.3/playbook/asciidoc-attributes/ (site.yml)
https://docs.antora.org/antora/2.3/component-attributes/ (antora.yml)

@EParzefall @phil-davis @jnweiger fyi