docs: when to use FastAPI-CacheX and when another library fits better

[allen0099]

Fixes #424

Summary

Adds a comparison page, docs/COMPARISON.md ("When to use it"), with a zh-TW copy. It covers fastapi-cache2, cashews, aiocache, and an HTTP cache or CDN in front of the app. For each one it says what it does well and when it is the better choice:

• cashews for function caching with early refresh, tags and metrics
• a CDN for public responses
• aiocache for a framework-independent cache

It also lists what FastAPI-CacheX does not do.

• The facts about the other libraries come from the source code of their latest PyPI releases: fastapi-cache2 0.2.2, cashews 7.6.0 and aiocache 0.12.3.
• Features that are not in a release yet (@cached, coalesce=True, HEAD from GET) are marked 0.4.2 (see Docs: readers on the PyPI release see behaviour from unreleased versions #330).
• The page is linked from both navs, the README and the zh-TW index.

Checklist

• The issue above is assigned to me (maintainer's own issue)
• Tests under tests/ cover the change: docs only, no code changes
• A changelog fragment: docs only, nothing under fastapi_cachex/
• uv run pre-commit run --all-files passes, and zensical build --strict passes for both languages

---

Generated by Claude Code