docs: translate the caching, backend and state guides into Traditional Chinese

[allen0099]

Part of #89 (PR 2 of 3: guides).

What

Five guides translated into i18n/zh-TW/docs/:

• HTTP caching (HTTP_CACHING.md)
• Cache flow (CACHE_FLOW.md)
• Application cache (APP_CACHE.md)
• Backends (BACKENDS.md)
• State (STATE.md)

The zh-TW nav and home page now link to these instead of the English site. Session, JWT claims, the API reference and the development pages still link to English, marked "（英文）".

Conventions

These are now recorded in i18n/zh-TW/GLOSSARY.md.

• Same anchors as English. Every translated heading carries its English anchor, e.g. ## 快取鍵 {#cache-keys}. Links such as CACHE_FLOW.md#2-cache-control-directives therefore work in both languages. For all five pages, the built heading ids are identical to the English site's.
• No hard-wrapped Chinese paragraphs. A newline between two CJK characters renders as a stray space in Chrome and Safari, so each paragraph is on one line. This PR also unwraps the home page.
• Glossary additions: the terms that came up while translating, e.g. 項目 (entry), 本文 (body), 重播 (replay), 行程 (process), 一次性 (one-shot), key builder and worker kept in English.

Code, identifiers, header and directive names stay unchanged; only code comments and the prose in the diagram blocks are translated.

Verified

• zensical build --strict and zensical build --strict -f zensical.zh-TW.toml both report no issues.
• Every translated page shows the "may be outdated" banner.