docs(cache): state that @cache goes below the route decorator

[allen0099]

Summary

With @cache written above @app.get(...), FastAPI registers the undecorated handler and never calls the cache wrapper. The route works, but nothing is cached and nothing warns. The docs never stated the required order.

• README quick start: an [!IMPORTANT] callout with a short ✅/❌ example, linking to the new section.
• docs/HTTP_CACHING.md: a new "Decorator order" subsection under "The @cache decorator". It explains why the order matters (decorators apply bottom-up, and FastAPI registers whatever reaches the route decorator), shows a ✅/❌ example, and notes that add_api_route must be given the decorated function.
• zh-TW mirrors in i18n/zh-TW/docs/index.md and i18n/zh-TW/docs/HTTP_CACHING.md ({#decorator-order} anchor, no line breaks inside Chinese paragraphs).

CACHE_FLOW.md has no usage section, so it is unchanged.

Why no runtime warning

The issue asked whether the reversed order could be detected at decoration time. No reliable, cheap signal exists:

• app.get(...) / router.get(...) return the function unchanged, with no attribute or marker. The only link is the APIRoute holding it, and @cache has no reference to any app or router, so it could only find one by scanning.
• gc.get_referrers(func) does find the APIRoute/Dependant, but it walks every GC-tracked object. That is tens of milliseconds per call on a large heap, and it would run for every decorated route at import time.
• It also has false positives. Registering a function uncached on one route and wrapping it with @cache for another route (app.add_api_route("/raw", f) plus app.get("/cached")(cache(ttl=60)(f))) is legitimate, but it looks identical to the reversed order.

Because a no-false-positive check is not possible, this PR is docs-only and has no changelog fragment.

Test plan

• uv run pytest -q --cov=fastapi_cachex
• uv run ruff check, uv run mypy fastapi_cachex --strict
• uv run pre-commit run --all-files
• zensical build for both the English and zh-TW sites; the callout and its code block render

Closes #324