[FEATURE]: Translate the V2 docs into Vietnamese (vi) #56316

The V2 docs under services/www/src/docs/content are English-only today. #55448 adds the multi-language layer (per-locale content directories plus a docsLocaleKeys registry) and currently ships ar and br. This issue tracks adding vi.

I have the Vietnamese translation of all 61 English pages ready, and will land it in five reviewable batches instead of one large PR:

Every batch only adds files under content/vi/, matching the layout #55448 uses, so nothing changes until the locale key exists. Once #55448 lands, enabling Vietnamese is a one-line addition to docsLocaleKeys.

Translation notes:

• One glossary across all 61 pages: agent → tác nhân, session → phiên, file → tệp, plugin/extension → tiện ích mở rộng, provider → nhà cung cấp, model → mô hình, prompt → lời nhắc, skill → kỹ năng, workspace → không gian làm việc. worktree stays in English, matching how the Vietnamese developer community uses it.
• Code, CLI flags, JSON keys, file paths, product names and keyboard legends stay verbatim; only prose is translated.
• Every page is checked against its English source for heading structure, byte-identical code fences, identical inline-code spans and link targets, table shapes, and leftover English prose.

Review from native speakers is welcome, especially for worktree, the free-model notes in the console pages, and the CLI/TUI terms.