Document multipurpose shared modules with native Swift packages

[Mark Turner (markst)]

Proposing a potential topic relating to building multipurpose shared modules. Something originally demonstrated in: https://github.com/markst/streamplayer-kt/

Summary

• Add an architectural topic on building multipurpose shared modules: a common Kotlin API with iOS implementation owned in Swift via swiftPMDependencies, consumable as both a KMP library and a standalone Swift package.
• Place the page under Explore project structure and cross-link from SwiftPM import, expect/actual, and project configuration.
• Cover dual Package.swift layout, team collaboration (Kotlin contract + thin bridge, Swift owns native behavior), dual-manifest alignment caveats, and community examples (streamplayer-kt, mapbox-maps-kmp, stripe-kmp).

Test plan

• Preview the new page in Writerside / docs build and confirm TOC entry Multipurpose shared modules appears under Explore project structure
• Verify internal links resolve (multiplatform-spm-import.md, multiplatform-expect-actual.md, multiplatform-project-configuration.md, multiplatform-spm-export.md, create-kotlin-multiplatform-library.md)
• Confirm Alpha / Kotlin version note and dual-Package.swift warning read correctly
• Spot-check community example links