Document createdById and createdAt on group memberships

[tommoor]

Summary

Reviewed PRs merged to outline/outline in the last day for changes to the public API. The relevant one is outline/outline#14000 ("Show who added a member in the group members table"), which added two fields to presentGroupUser:

• createdById — the user who added the member to the group
• createdAt — when the membership was created

These feed the group-member endpoints (groups.memberships, groups.add_user, groups.update_user, and the membership preview in groups.list), all of which reference the shared GroupMembership schema. This PR adds both fields to that schema.

Only spec3.yml was edited by hand; spec3.json was regenerated by the pre-commit hook.

Other merged PRs reviewed (no API impact)

• #13876 (link breadcrumbs), #13993/#13786 (sidebar), #13994/#14006 (unsynced-changes), #13997/#14003/#14004 (editor), #14001/#14005 (Sentry) — frontend/editor/internal only, no API schema or output-shape change.
• #13998 ("Emit events when deleting synced groups") — behavioral change to groups.deleteAll (an admin endpoint not in the spec); emits delete events, no input/output shape change.
• #13945 (private-use OAuth redirect URIs) — already documented in Document private-use scheme redirect URIs on OAuth client endpoints #65.
• Dependabot PRs ignored per task guidance.

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