docs(adr): a module for signing in with OpenID Connect, SimpleLogin a preset (ADR 0023)

The developer's decisions of 23 Oct: the sign-in of ADR 0020 becomes a
Serverpod module others can use, for any OpenID Connect provider with
SimpleLogin as a preset, under BSD-3-Clause, in this repository until it
runs on learn.halkyon.eu, built now as ADR 0020's step 2. Serverpod v4.0.4
has a contract for providers of one's own but nothing of OpenID Connect.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
Signed-off-by: Tizian 「ティツィアーン」 <raisondetredev@pm.me>

docs/adr/0020-sign-in-with-simplelogin.md

@@ -93,7 +93,8 @@ against app.simplelogin.io on 23 Oct:
   `kid` names, never fetches the keys again for an unknown `kid` (an hour of failed
   sign-ins after SimpleLogin changes its key) and fetches them without a timeout. So:
   `checkHeaderType` off, `RS256` alone, the key the `kid` names, fetched again for one
-  unknown, with a timeout (over adapting a copy of Serverpod's verifier, or `jose`).
+  unknown, with a timeout (over adapting a copy of Serverpod's verifier, or `jose`); in
+  the OpenID Connect module the sign-in is built on (ADR 0023).
 - **D13 Attempts in a table** (recommended, over the server's memory): a restart or a
   deploy in the middle of a sign-in loses none, and the database takes each once.
 - **D14 Steps 2 to 4 in one release** (recommended, over the server's side alone first):