Add full English version of API reference docs under docs/en/api/,
covering 69 files across kernel, framework, and network modules.
Additional changes in docs/zh-cn/api/:
- Add bilingual navigation header to all 69 zh-cn API documents,
enabling cross-language switching between Chinese and English versions.
- Fix 16 mis-placed function descriptions in telephony/telephony_call.md
where multiple APIs incorrectly shared the same description
(re-derived from the tapi_call.h header @brief comments).
- Fix 2 untranslated English residues in network/wapi.md.
zhangxiaowei16