Documentation, R-CMD Check workflows & Remove rgpt3 #38

[jan-abel-inwt]

• [x] https://github.com/Pandora-IsoMemo/drat/issues/33
• [x] https://github.com/Pandora-IsoMemo/drat/issues/35
• [x] https://github.com/Pandora-IsoMemo/drat/issues/34
• [x] https://github.com/Pandora-IsoMemo/drat/issues/36
• [x] https://github.com/Pandora-IsoMemo/drat/issues/37