Docs:
MitID Documentation : MitID | Developer Pages
Signature Documentation: Electronic signatures | Developer Pages
DocumentService API (SOAP) : DocumentService v3 | Developer Pages
Tools:
MitID test tool : MitID Test Tool
Customer Portal login (only for going to prod): https://customerportal.signicat.com/
Other:
Information from Digitaliseringsstyrelsens webpages : MitID