Ledger config: accept a string literal for f:ledger #2066

f:ledger in ledger configuration is read only as an IRI reference. A ledger id with a slash (org/governance:main) has no absolute IRI spelling, so documented cross-ledger config writes it as a relative reference (f:ledger <org/governance:main>), which TriG GRAPH blocks keep verbatim only because #2008 preserves that behavior for them.

Accepting a string literal there (f:ledger "org/governance:main"), and documenting it, would let config avoid relative references.

Found while reviewing #2008 (listed in its follow-ups).