Generate ASCIIDoctor PDF during CI pipeline

Remco de Manrequested to merge
fbl_ci_docs_generation into master

This merge request enables the compilation of an ASCIIDoctor PDF of the owners manual during the CI pipeline. This allows for downloading a PDF version of the manual for each pipeline.

Furthermore, the PDF version of the owners manual is now also published to GitLab pages when a new version is released.

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