Added index4.html, reflecting latest F2F version 

and added implementation feedback. Added diagrams in LibreOffice format.
index4.html serves as staging version for this review. If accepted, becomes the main version of the specification, with the Changes section updated as follows:
- removed separate events for call list changes, as it duplicated functionality from state changes. Call state changes are sufficient to handle all use cases of call life cycle management
- simplified conference call handling
- made the telephony service related parts informal, and "invisible" by default
- updated state transition figures
- reworked/checked the algorithmic steps.