Adds a code sample and a code walkthrough explaining how to build a
service that issues Credentials (XLS-70) on the XRP Ledger.
Credential issuer: Clarify/revise documents field
Issue credentials code sample: fix bugs
Apply suggestions from @oeggert review
Co-authored-by: oeggert <117319296+oeggert@users.noreply.github.com>
Credential Issuer: more edits for clarity
