You can register an account with an email address and password or with itsme®.
If you would like to use eID on Doccle, you can link your eID card after registration, via Preferences => Security.
Would you rather not use a card reader? With itsme® you are at least as safe and you only need your smartphone. You can also directly create your new account with itsme and log in with it in Doccle's mobile app.