Add Credential Issuer tutorial

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

_code-samples/issue-credentials/py/decode_hex.py

@@ -0,0 +1,14 @@
+from binascii import unhexlify
+
+def decode_hex(s_hex):
+    """
+    Try decoding a hex string as ASCII; return the decoded string on success,
+    or the un-decoded string prefixed by '(BIN) ' on failure.
+    """
+    try:
+        s = unhexlify(s_hex).decode("ascii")
+        # Could use utf-8 instead, but it has more edge cases.
+        # Optionally, sanitize the string for display before returning
+    except:
+        s = "(BIN) "+s_hex
+    return s