docs: render missing docs for scheduled tasks admin api (#18516)

Co-authored-by: Andrew Morgan <1342360+anoadragon453@users.noreply.github.com>

Author: dklimpel

changelog.d/18516.doc

@@ -0,0 +1 @@
+Surface hidden Admin API documentation regarding fetching of scheduled tasks.

docs/SUMMARY.md

@@ -68,6 +68,7 @@
       - [Registration Tokens](usage/administration/admin_api/registration_tokens.md)
       - [Manipulate Room Membership](admin_api/room_membership.md)
       - [Rooms](admin_api/rooms.md)
+      - [Scheduled tasks](admin_api/scheduled_tasks.md)
       - [Server Notices](admin_api/server_notices.md)
       - [Statistics](admin_api/statistics.md)
       - [Users](admin_api/user_admin_api.md)