Add Aspire Skills release note

[IEvangelist]

Summary

• Add an Aspire 13.4 release-note section for AI agent readiness and the new microsoft/aspire-skills bundle.
• Explain why the three focused skills (aspire-init, aspire-orchestration, and aspire-monitoring) exist and what each is good at.
• Link readers to the AI coding agents guide and the microsoft/aspire-skills repository.

Validation

• pnpm --dir .\src\frontend run build:skip-search
• pnpm --dir .\src\frontend exec prettier --check .\src\content\docs\whats-new\aspire-13-4.mdx --plugin prettier-plugin-astro
• git --no-pager diff --check