docs(cs): add Czech (Čeština) documentation translation

[JakubStejskalCZ]

Feature Request

What: Add a complete Czech (Čeština) translation of the BMad Method documentation.

Why: Expand BMad's accessibility to Czech-speaking developers and the Czech tech community. This follows the same pattern established by the existing French and Chinese translations.

How it strengthens human-AI collaboration: Lowering the language barrier allows more developers to adopt BMad's guided workflows and agent-based development practices.

Scope:

• 29 translated documentation files in docs/cs/
• Locale registration in locales.mjs (cs-CZ)
• Sidebar translations in astro.config.mjs
• Full parity with English docs: tutorials, how-to guides, explanations, and references

[coderabbitai]

🔗 Related PRs

#1822 - feat(i18n): add zh-CN locale support [merged]
#1848 - docs: fix chinese documentation translation errors [merged]
#1920 - docs(zh-cn): add translation for quick-dev-new-preview [merged]
#2002 - docs(zh-cn): add Chinese translation for core-tools reference [merged]
#2091 - fix(docs): community feedback — typo, locale 404s, llms-full [merged]

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