docs: developer_guides: add how-to guide on customizing multi-slot wov and ecns pipelines

Add a comprehensive developer guide under Developer Guides Section 1
(Firmware Development) detailing the architecture and step-by-step
customization of complex multi-slot Wake-on-Voice (WOV) and Echo
Cancellation & Noise Suppression (ECNS) pipelines based on the
wcl-uaol-wov branch:

- Document the 4-channel native 16 kHz DMIC dual-stream architecture
  (ECNS communication stream on PCM 10 and multi-slot WOV keyword/audio
  capture stream on PCM 11).
- Detail the lockless, zero-IPC SOF Notifier event bus contracts
  (NOTIFIER_ID_KPB_CLIENT_EVT, NOTIFIER_ID_WOV_DETECT, and
  NOTIFIER_ID_WOV_CTRL) for inter-module arbitration.
- Provide step-by-step implementation templates for replacing the ECNS
  module (multi-pin audio contract: mono clean speech to KPB, stereo
  clean speech to Host Copier 10) and replacing keyword spotters.
- Explain ALSA Topology 2.0 scheduling domains: Low-Latency (LL, 1ms,
  lp_mode 0) vs. Data Processing (DP, 10ms/20ms, lp_mode 1), mathematical
  buffer sizing rules (IBS/OBS), and multi-core affinity (core_id).
- Provide topology compilation, NHLT ACPI BIOS override instructions
  (sof_use_tplg_nhlt=1), hardware capture verification, and a developer
  watch-out checklist.
- Include two detailed SVG diagrams illustrating the 8-pipeline
  signal flow architecture and the multi-period scheduling timeline.

Signed-off-by: Liam Girdwood <liam.r.girdwood@linux.intel.com>

Author: lrgirdwo