Describe Digdir's classification guideline

https://pedia.altinn.cloud/sec/security-architecture/data-security/data-classification/ (or a page referred to from this page) should be updated with information on Digdir's classification guideline. Once approved, relevant parts should be migrated from pedia to docs.

Start with the linked data-classification page and any page it refers to, then review Digdir's classification guideline to identify the relevant information. Update the Pedia content for review and approval, and migrate the approved sections to the Altinn docs. Done means the guideline is accurately described in the approved documentation and the migrated content is available in docs.

Written by the indexing model from the issue text.