Document a monorepo configuration example

[Hayal08]

Goal

Add a practical documentation example for teams adopting PR Sheriff in a small monorepo.

Suggested scope

• Show a repository with two packages and shared infrastructure.
• Explain test patterns, ignored docs, and sensitive paths.
• Explain current limitations before first-class monorepo support exists.

Acceptance criteria

• Add a focused guide under docs/.
• Include a complete copyable .pr-sheriff.json example.
• Link the guide from README and ROADMAP.
• Do not claim unsupported package-level risk scoring.

Getting started

This is a documentation-only task. Use the current configuration keys and be explicit about limitations.