R Lernen landing page

[pr130]

Once R Lernen is split into separate apps, we could add a landing page for R Lernen on the website directly. see this issue: CorrelAid/lernplattform#15

Timeline for this issue probably 2-3 months (~may 2023)

but in general, this is tbd as other models (e.g. a separate subdomain rlernen.correlaid.org) would also be beneficial/nice.