Add ability to create commitment certifications
This depends on [gitlab.com:schwalbe-hd/schwalbe#259](https://gitlab.com/schwalbe-hd/schwalbe/-/issues/259): As soon as this is implemented, there is a standalone `pdf_generator` package we can use to generate certifications similar to the ones created by Schwalbe (see [example](https://gitlab.com/schwalbe-hd/schwalbe/-/raw/master/screenshots/certification-of-commitment.png)).
This would need several steps:
* [x] Add `pdf_generator` as dependency
* [ ] Create custom Coli CSS modifications
* [x] Create a set of Coli templates (HTML) and corresponding certification classes (or find something dynamic for the latter)
* [x] Document how to create such a template
* [x] Add a converter function similar to the [main implementation in Schwalbe](https://gitlab.com/schwalbe-hd/schwalbe/-/blob/5d8d899a0afdd5eb872f6c41e51380b6b9d0fa53/pdf_generator/commitment_certification/__main__.py#read-only-access-to-individual-data)
The API should the look like this:
1. Create a JSON file containing one or more certification requests (maybe also just one)
2. Convert this to PDF (or maybe HTML) with the Coliverter
3. Enjoy your beautiful commitment certifications
issue