Add support for credit class document field

[blushi]

Is your feature request related to a problem? Please describe.
regen-network/rnd-dev-team#1643 was expecting a credit class document to be part of the credit class data to display some "view credit class doc" button but we don't really know yet what the field name in class metadata will be (or even if it's relevant for existing classes at this point).

Describe the solution you'd like
Add support for the credit class doc once we have the field name.
As part of #1970, support for displaying the credit class doc button was already added so we would just need to get it from the class metadata and provide it as DetailsSection component prop.

Additional context (optional)
#1970 (comment)

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[ryanchristo]

Pending document from registry team.