chore(data): Move snowball DOIs into Zotero

Integrated all new snowballing DOIs into Zotero to be screened and
extracted.

02-data/raw/02_snowballing-starts/README.md

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+# Snowballing starters
+
+These directories contain the references and DOIs for studies that started the snowballing process.
+
+Each directory is named after a (usually systematic review) study which is used to start the process, with the same citation key as in the Zotero sample pool. 
+
+The directories contain a `references.txt` file which contains all relevant references mentioned in the study, enumerated. 
+Usually this will be either *all* references used in the study or only those included in the review itself, if mentioned separately.
+
+The directories also contain a `DOIs.txt` and `DOIs_only.txt` file, 
+which contain the aforementioned references paired with DOI numbers where available and the DOI numbers only, respectively.
+
+Finally, each directory contains an empty `<num>-of-<num>` file with the placeholders representing two numbers.
+The numbers point out the total amount of references for this study (the last), 
+as well as the number of DOIs able to be extracted for them (the first).
+
+A few directories are doubled, and one will usually carry the `StudyYear` name while the other will be `StudyYear_included`.
+For these, the `StudyYear` directory contains references for *all* references mentioned in the study, while the `StudyYear_included` directory only contains references actually used in the review study.