Replace spin-off branches with different build options

[bkimminich]

Maintaining these three different branches is either a tedious effort or some get less attention and "die off" over time. To amend this problem there should be only one branch again.

1. webinar can be eliminated entirely if exercises are phrased neutral for on-site/remote usage
2. workshop would require a way to build a separate deck from the source MDs where some slides are skipped during PDF creation