docs: support i18n and add Simplified Chinese (zh-CN) translation for A2UI documentation

[Thulof]

Description

This PR adds full Simplified Chinese (zh-CN) translation support for the A2UI documentation.

Changes

• Translation: Added translated markdown files in docs/zh/ mirroring the structure of the original docs/.
• Configuration: Updated mkdocs.yaml to enable the zh locale using mkdocs-static-i18n plugin and added a localized navigation bar (nav_translations).
• Specification Sourcing: The translated specification files (docs/zh/specification/*.md) directly reference the original English specification files (e.g., in specification/v0_8/docs/) using the snippet inclusion syntax (--8<--). This ensures the technical specification remains a single source of truth while the wrapper content is localized.

Pre-launch Checklist

• [x] I signed the CLA.
• [x] I read the Contributors Guide.
• [x] I read the Style Guide.
• [x] I have added updates to the CHANGELOG.
• [x] I updated/added relevant documentation.
• [x] My code changes (if any) have tests.

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