Add structured docs/user-guide set for Digital Signage and wire README to docs entrypoint

[Copilot]

This PR introduces a dedicated docs/user-guide documentation tree for this repository, aligned with the structure used in the referenced Open Edge Platform projects. Existing content in README.md is reorganized into task-oriented docs pages (get started, system requirements, how-to guides, API reference, troubleshooting, and release notes).

• Documentation scaffold

  • Added docs/user-guide/index.md as the documentation entrypoint.
  • Added top-level navigation pages: get-started.md, how-it-works.md, how-to-guides.md, api-reference.md, troubleshooting.md, and release-notes.md.

• Get-started breakdown

  • Added:
    • get-started/system-requirements.md
    • get-started/build-from-source.md
    • get-started/deploy-with-docker-compose.md
  • Split setup guidance into requirement, build, and deployment paths for faster onboarding.

• Operational how-to content

  • Added focused guides under how-to-guides/ for:
    • predefined ad configuration
    • simulation video switching
    • RTSP camera setup
    • Intel® Geti™ model usage
    • inference device switching (CPU/GPU/NPU)

• Release documentation

  • Added release notes index and initial versioned notes:
    • release-notes/release-notes-2026.md

• README integration

  • Added a direct link from README.md to docs/user-guide/index.md to route readers to the structured docs set.

Example of the new docs navigation shape:

# Digital Signage

## User Guide
- [Get Started](./get-started.md)
- [How It Works](./how-it-works.md)
- [How-to Guides](./how-to-guides.md)
- [API Reference](./api-reference.md)
- [Troubleshooting](./troubleshooting.md)
- [Release Notes](./release-notes.md)