feat(instances): document calendar event linking

[BASIC-BIT]

Summary

• document PUT /instances/{worldId}:{instanceId} as updateInstance
• add UpdateInstanceRequest with nullable calendarEntryId for linking and unlinking calendar events
• extend the existing group-calendar lifecycle to create a members-only instance, verify linking and unlinking, close the instance, and retain the existing event/group cleanup

Closes #596.

[pkg-pr-new]

pnpm add https://pkg.pr.new/vrchatapi/specification/vrchat@accb0a4

commit: accb0a4