Redesign docs around decoupled optics/sensor/network architecture

[Copilot]

DeepLens documentation still described an older package layout and made the architecture harder to parse after the codebase split into three decoupled parts. This PR updates core docs to present the system as optics + sensor + network, with Camera as the composition layer.

• Architecture narrative (user guide)

  • Reworked docs/user_guide/architecture.rst overview to center on the three decoupled modules:
    • deeplens.optics
    • deeplens.sensor
    • deeplens.network
  • Clarified deeplens.camera.Camera as the wrapper that composes optics and sensor simulation.
  • Added a dedicated Network section to complete the module-level architecture story.

• Repository structure docs (top-level + quickstart)

  • Updated structure blocks in:
    • README.md
    • docs/quickstart.rst
  • Removed outdated references to lens files at deeplens/ root and replaced with the current module-first layout.
  • Explicitly documented camera.py as the composition entry point.

• Readability/consistency cleanup

  • Aligned terminology across docs to consistently use the decoupled module framing.
  • Tightened section wording to reduce duplication and improve scanability.

deeplens/
├── optics/   # optical simulation and lens models
├── sensor/   # sensor and ISP simulation
├── network/  # surrogate + reconstruction networks
└── camera.py # composition wrapper (optics + sensor)

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