feat(docs): Spanish (es) translations

[vishr]

What

Adds Spanish (es) as a fourth locale alongside English, 简体中文, and 日本語. Full translation pass of the Guide, Middleware, and Cookbook sections (59 docs), done via Codex.

Changes

• astro.config.mjs — register the es locale ({ label: 'Español', lang: 'es' }) and add es sidebar group-label translations (Guía / Middleware / Recetario).
• src/content/docs/es/** — 59 translated docs mirroring the en structure (guide, middleware, cookbook).

Verification

• Merged current master into the branch first — clean merge, coexists with the "Ask AI" pill work from feat(docs): themed 'Ask AI' header pill (kapa.ai) #413 (terminal.css pill rules + es locale both present).
• Production build passes: 241 pages, including 60 es/ HTML pages in dist.
• Translation integrity checks: all code fences balanced (no broken code blocks), no un-localized internal links, frontmatter title/description translated to Spanish.
• Sidebar es: translation keys correctly use the BCP-47 lang tag (lang: 'es'), matching the existing zh-CN/ja convention.

🤖 Generated with Claude Code