Add documentation for creating custom forecast types

[seabbs-bot]

Problem

The package supports extensibility through S3 dispatch but there is no documentation covering how to create a custom forecast type. The README mentions users can "extend existing classes to handle new types of forecasts" and the manuscript acknowledges this gap, noting that "a third area of improvement is the addition of case studies and vignettes that make working with and extending functionality from the package easier."

Existing documentation

• Custom scoring functions are well covered (vignettes, score() roxygen, manuscript).
• Custom forecast types are not documented at all.

What's needed

A vignette or article explaining the steps to create a new forecast type, including:

• Creating as_forecast_<type>() constructor (generic + default method)
• Implementing assert_forecast.<type>() for validation
• Implementing score.<type>() for scoring
• Implementing get_metrics.<type>() for default metrics
• Implementing get_forecast_type_ids.<type>() for duplicate detection (after Rework get_duplicate_forecasts() to S3 to avoid hard-coding columns and types #888)
• Registering S3 methods in NAMESPACE
• Working example of an end-to-end custom type

This was opened by a bot. Please ping @seabbs for any questions.

[nikosbosse]

Agreed. Should we have @seabbs-bot produce a draft?