Add Aspire Skills documentation

[IEvangelist]

Summary

• Add a dedicated Aspire Skills docs page covering the microsoft/aspire-skills repo, included skills, installation paths, project-local precedence, and plugin guardrails.
• Cross-link the existing AI coding agents and dashboard agent docs, and add the page to the AI coding agents sidebar section.
• Add microsoft/aspire-skills to GitHub stats and the community contributors page.
• Remove the soon-to-be-deleted Aspire Skills public site link and remove MCP references from the affected agent guidance.

Validation

• pnpm --dir .\src\frontend exec prettier --check --plugin prettier-plugin-astro .\src\content\docs\get-started\aspire-skills.mdx .\src\content\docs\get-started\ai-coding-agents.mdx .\src\content\docs\dashboard\ai-coding-agents.mdx
• pnpm --dir .\src\frontend run test:unit:docs
• pnpm --dir .\src\frontend exec vitest run --config vitest.config.ts tests/unit/seo-lengths.vitest.test.ts