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/accept_credential.py

@@ -0,0 +1,61 @@
+#!/usr/bin/env python
+
+from binascii import unhexlify
+from os import getenv
+from getpass import getpass
+
+from xrpl.clients import JsonRpcClient
+from xrpl.models.requests import AccountObjects, AccountObjectType
+from xrpl.models.transactions import CredentialAccept
+from xrpl.transaction import submit_and_wait
+from xrpl.utils import str_to_hex, datetime_to_ripple_time
+from xrpl.wallet import Wallet, generate_faucet_wallet
+
+from look_up_credentials import look_up_credentials
+from decode_hex import decode_hex
+
+XRPL_SERVER = "https://s.devnet.rippletest.net:51234/"
+
+client = JsonRpcClient(XRPL_SERVER)
+
+def init_wallet():
+    seed = getenv("SUBJECT_ACCOUNT_SEED")
+    if not seed:
+        seed = getpass(prompt='Subject account seed: ',stream=None)
+    if not seed:
+        print("Please specify the subject's master seed")
+        exit(1)
+    return Wallet.from_seed(seed=seed)
+
+wallet = init_wallet()
+
+pending_credentials = look_up_credentials(
+        client, 
+        subject=wallet.address, 
+        accepted="no"
+)
+
+prompt = """
+Accept a credential?
+    0) No, quit."""
+for i, cred in enumerate(pending_credentials):
+    credential_type_s = decode_hex(cred["CredentialType"])
+    prompt += f"\n    {i+1}) '{credential_type_s}' issued by {cred['Issuer']}"
+
+selection = None
+options = [str(n) for n in range(len(pending_credentials)+1)]
+while selection not in options:
+    selection = input(prompt+f"\n Select an option (0-{len(options)-1}): ")
+
+if selection == "0":
+    exit(0)
+
+chosen_cred = pending_credentials[int(selection)-1]
+tx = CredentialAccept(
+    account=wallet.address, 
+    credential_type=chosen_cred["CredentialType"],
+    issuer=chosen_cred["Issuer"]
+)
+print("Submitting transaction", tx)
+response = submit_and_wait(tx, client=client, wallet=wallet, autofill=True)
+print(response)