docs: add comprehensive development setup instructions to CONTRIBUTING.md (H7) (#8860)

* docs: add comprehensive development setup instructions to CONTRIBUTING.md

- Expand prerequisites with Go version requirements and installation links
- Add system dependencies for Ubuntu/Debian, CentOS/RHEL/Fedora, macOS, and Windows
- Document build commands with explanations and key build variables
- Add environment variables section with useful development env vars
- Include development workflow guidelines (branch naming, commit format, PR process)
- Enhance testing section with per-package and focused test instructions

* Apply suggestions from code review

Signed-off-by: Ettore Di Giacinto <mudler@users.noreply.github.com>

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Signed-off-by: Ettore Di Giacinto <mudler@users.noreply.github.com>
Co-authored-by: localai-bot <localai-bot@noreply.github.com>
Co-authored-by: Ettore Di Giacinto <mudler@users.noreply.github.com>