Add a third-party check plugin authoring guide and reference example

[parthdongre]

Summary

Document the existing third-party check discovery mechanism and provide a minimal reference plugin that contributors can copy.

Why

FrameVitals already supports custom checks and opt-in third-party check discovery through Python entry points. That is an important extensibility feature, but ecosystem growth depends on making the extension contract obvious to outside developers.

Scope

• Document the plugin/check entry-point contract.
• Show the smallest possible external package layout.
• Explain check naming, severity, return values, errors, and compatibility expectations.
• Add a reference example under examples/ or a dedicated docs page.
• Add a lightweight integration test proving an installed example plugin can be discovered and executed.

Acceptance criteria

• A contributor can build a working external check plugin from the docs alone.
• Entry-point configuration is shown for modern Python packaging.
• Expected check result schema and error behavior are documented.
• A minimal integration test covers discovery + execution.
• The guide links from CONTRIBUTING.md or the main docs navigation.

Priority: P2 — ecosystem / contributor experience
Suggested labels: documentation, plugins, help-wanted