Use roxygen markdown for help documentation

[andrie]

This changes the help documentation to use roxygen markdown, since this is in general easier to write and maintain.

In addition, moves documentation examples into inst/examples/___.R - this makes it easier to edit documentation examples, since you can run the R code directly.

DoD

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• [x] Change has been tested (manually or with automated tests), everything runs correctly and works as expected. No existing functionality is broken.

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• [x] All interaction with a semantic functions, examples and docs are written from the perspective of the person using or receiving it. They are understandable and helpful to this person.

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