Add "Multiple API Versions" pattern and restructure patterns index

[Copilot]

Summary

Adds a new $.minVersion() pattern page and rewrites the patterns index introduction, which had grown into a single unwieldy run-on paragraph.

docs/patterns/multiple-versions.md (new)

• Problem/Solution/Example/Consequences/Related Patterns structure matching existing pattern pages
• Shows how to configure multiple versioned specs under one group using counterfact.yaml
• Demonstrates layered $.minVersion() branching in a single shared handler across v1/v2/v3
• Covers TypeScript type-narrowing after a minVersion check

docs/patterns/index.md

• Replaces the single 300-word paragraph with six labelled sections: Getting started, Organizing state, Runtime control, Versioned and multi-spec APIs, Long-term reliability, Integration strategies
• Adds Multiple API Versions row to the All patterns table

Original Prompt

Create a pattern that describes working with multiple versions of an API and using $.minVersion().

Also rewrite the patterns index introduction. It's gotten very long. It should be broken up into multiple paragraphs, and possible even have section headers.

Manual acceptance tests

• docs/patterns/multiple-versions.md renders correctly and all Related Patterns links resolve to existing files
• docs/patterns/index.md introduction displays as distinct labelled sections rather than one long paragraph
• "Multiple API Versions" row appears in the All patterns table and its link navigates to the new page
• All pre-existing pattern links in index.md still resolve correctly
• The $.minVersion() example in the new pattern matches the runtime semantics: true when current version ≥ min in declaration order, absent on unversioned runners
• The counterfact.yaml configuration example in multiple-versions.md uses the correct YAML spec array format with source, group, and version keys

Tasks

• Created docs/patterns/multiple-versions.md with full pattern structure covering configuration (via counterfact.yaml), runtime helpers, TypeScript narrowing, and consequences
• Rewrote docs/patterns/index.md introduction into six sectioned paragraphs; added new pattern to All patterns table
• Added .changeset/multiple-versions-pattern.md (patch bump)
• Updated configuration example in multiple-versions.md to use counterfact.yaml YAML format instead of the programmatic counterfact() API

[pmcelhaney]

Explain in terms of the config file, not the API.

[Copilot]

Done in 4e3d1d2. The Solution section now says to list specs under the spec key in counterfact.yaml, and the Configuration example is a YAML block instead of a TypeScript counterfact() call.