Add a FAIR data box to the Feasibility section (#61)

[LukasWallrich]

Adds a callout box on FAIR data to the Feasibility section of the Choosing the Target Study chapter.

The box does two things:

1. Explains what FAIR means (findable, accessible, interoperable, reusable) in operational terms: persistent identifier, stated access conditions, a codebook, non-proprietary file formats, an explicit license, and why this makes a reproduction feasible with little involvement from the original authors.
2. Turns the criteria around into a search strategy: start from well-documented open datasets and locate the papers built on them. Named examples are the Open Science Framework, Harvard Dataverse, the GESIS Data Archive, ICPSR, the UK Data Service, and the economics database compiled by Sebastian Kranz, which the handbook already cites in the Execution of Reproductions chapter.

The FAIR sentence that was in the preceding paragraph now points to the box, so the definition is given once.

No new references were added; both citations in the box (@WilkinsonEtAl2016, @KranzDataList) are already in references.bib. The chapter renders without citation or cross-reference warnings.

Closes #61