Closes #221: Feature request - Provide CSV files

[Gero1999]

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Implementation description

This pull request introduces the automated export of SDTM datasets to CSV format and adds two example CSV files to the repository. The changes improve reproducibility and make example datasets more accessible for external use or testing.

• The data generation script (data-raw/create_sdtms_data.R) now saves a CSV version of each dataset to the inst/extdata/ directory, making the datasets directly available for non R-programmers.

• The .Rbuildignore file is updated to exclude all CSV files in inst/extdata/ from the R package build, ensuring these files are not included in the built package by default for CRAN submissions.

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