doc: api: ipc4: document protocol architecture and integrate all 8 IPC4 subgroups

[lgirdwood]

Summary

Overhauls the IPC4 API documentation in api/ipc4.rst with comprehensive protocol architecture documentation, ASCII flow diagrams, and integration of all eight new IPC4 Doxygen subgroups:

1. ipc4_envelope_api: 64-bit Message Envelope & Handlers
2. ipc4_format_api: Audio Formats, Pins & Stream Descriptors
3. ipc4_module_api: Dynamic Modules & Configuration Handshake
4. ipc4_pipeline_api: Audio Pipelines & Scheduling Priorities
5. ipc4_base_fw_api: Base Firmware & ADSP Configuration
6. ipc4_gateway_api: Hardware Gateways & Peripherals
7. ipc4_notification_api: Asynchronous Notifications & Mailbox Registers
8. ipc4_telemetry_probe_api: Telemetry, Probes & Logging

Included Protocol Explanations & Diagrams

• Shared SRAM Mailbox Windows 0 & 1 layout and register mappings.
• 64-bit Message Envelope (Primary & Extension DWORD bitfields).
• Hardware Doorbell handshake flow (Host BUSY / DSP DONE).
• Dynamic audio pipeline construction, module initialization, inter-module binding, and state machine ordering.
• Multi-chunk Large Configuration protocol sequence.

Verification

• Clean build with SPHINXOPTS="-W --keep-going" make html passing with 0 errors and 0 warnings.
• Tested against the updated SOF Doxygen XML tree from doc: ipc4: comprehensive audit and upgrade of IPC4 protocol Doxygen documentation sof#11302.