docs: improve use package docs

[aose-yuu]

Purpose

Make the use package metadata and docs match the current Sigrea API and remain readable from npm.

Summary

• Clarifies the package description and keywords around framework-agnostic Sigrea utilities.
• Restores and links the @sigrea/core relationship in the README.
• Replaces npm-broken relative documentation links with GitHub URLs.
• Updates the createEvents molecule example to use props parameter typing so return types stay inferred.
• Wraps long README prose around sentence and clause boundaries.

Verification

• pnpm -s cicheck