chore: Halkyon Learn moves into apps/learn

Its server, client, app and curator, and the tools that write its
assets (maps, Anki fixtures, icons), as they are; paths are fixed in
the commits that follow.

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

apps/learn/halkyon_learn_client/doc/endpoint.md

@@ -0,0 +1,15 @@
+# Callable endpoints
+
+Each class contains callable methods that will call a method on the server side. These are normally defined in the `endpoint` directory in your server project. This client sends requests to these endpoints and returns the result.
+
+Example usage:
+
+```dart
+// How to use GreetingEndpoint.
+client.greeting.hello("world!");
+
+// Generic format.
+client.<endpoint>.<method>(...);
+```
+
+Please see the full official documentation [here](https://docs.serverpod.dev)