feat: add changelog text to Modrinth uploads

[DriftN2Forty]

Summary

• add a workflow step that extracts the correct changelog text before Modrinth publishing
• publish the matching release section for main builds and the [Unreleased] section for dev alpha builds
• document the workflow change in the unreleased changelog

Validation

• verified the changelog extraction logic locally for a release version (1.4.0)
• verified the changelog extraction logic locally for a dev alpha version (1.4.0-alpha.123)
• confirmed the workflow file has no YAML errors in the editor