docs: show what each Cache-Control directive actually does

[allen0099]

Closes #143

Changes

• docs/HTTP_CACHING.md: the Cache-Control table now has these columns: directive, how to set it, whether it is sent in the header, and its effect on the server-side cache.
  • public, immutable, must-revalidate, stale-while-revalidate and stale-if-error are marked header-only.
  • The directives that @cache cannot emit are listed with no setting.
  • A short paragraph explains that no_cache/no_store replace the rest of the header, and links to Cache flow.
• New subsection, "The request's Cache-Control is ignored": it documents that client request directives such as no-cache and max-age=0 do not bypass the cache. This is by design, because otherwise any client could send every request to the handler. If-None-Match is still honoured.
• docs/README.zh-TW.md: the same table and note, in Traditional Chinese. The claim that the directives are completely implemented ("完整實現 Cache-Control 指令") is replaced.
• README.md: "comprehensive HTTP caching support" becomes a concrete description.
• CHANGELOG: entry under Fixed.

Adding the directives that @cache cannot emit yet is tracked in #144.

Verification

• zensical build --strict passes, and the table renders with all 13 directives.
• I checked the documented behaviour against a running app:
  • Cache-Control: no-cache and max-age=0 request headers still get the cached response.
  • no_store=True responses carry no ETag.