Docs: Add CONFIG.md documenting config files, env vars, and precedence

[GaneshPatil7517]

Closes #503

Adds docs/CONFIG.md a single reference document for concore's configuration system. No code changes.

What's Documented

• concore.tools all 14 supported keys (CPPEXE, PYTHONEXE, VEXE, etc.) with format, platform variants, and examples
• concore.octave presence-based flag to treat .m files as Octave
• concore.mcr MATLAB Compiler Runtime path
• concore.sudo Docker executable override (sudo docker, podman, etc.)
• concore.repo Docker Hub repository prefix override
• Environment variables all 15 CONCORE_* and DOCKEREXE variables with defaults
• Precedence order concore.tools > concore.sudo > env vars > hardcoded defaults
• Quick start examples minimal, typical, Windows, Docker, and Octave setups

Why
Per mentor feedback on #503: "If your goal is to simply document these four files somewhere handy, go for it." This is purely additive documentation no code, no config format changes, no modifications to mkconcore.py.

Verified

• All 170 tests pass (pytest tests/ -v)
• ruff format --check clean
• All information sourced directly from mkconcore.py

[Copilot]

Pull request overview

Adds a new single-source reference document for concore’s configuration system as implemented by mkconcore.py, covering supported config files, environment variables, and resolution/precedence rules.

Changes:

• Introduces docs/CONFIG.md documenting CONCOREPATH resolution and supported legacy config files (concore.tools, concore.octave, concore.mcr, concore.sudo, concore.repo).
• Documents tool key/variable mappings for compiler/runtime selection across platforms.
• Adds a stated precedence order and quick-start configuration examples.

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