docs: add English translation for API reference documentation

[tanghao-xiaomi]

Summary

This PR adds a complete English version of the openvela API reference documentation and improves the existing Chinese version.

Changes

Added

• docs/en/api/ — English translation of the API reference, 69 files in total, covering the kernel, framework, and network modules.
• A language switch header is added at the top of every English file: \[ English | [简体中文](...) \]

Modified (docs/zh-cn/api/)

• Added a language switch header \[ [English](...) | 简体中文 \] to all 69 Chinese files, enabling bidirectional navigation between the two languages.
• framework/telephony/telephony_call.md: Fixed 16 mis-placed function descriptions. In the original Chinese document, multiple unrelated APIs incorrectly shared the same description (a batch-copy mistake). Descriptions have been re-derived from the @brief comments in the tapi_call.h header, and the English version has been updated in sync.
• network/wapi.md: Fixed 2 places where English text had been left untranslated.

Coverage

• kernel: mem, sched, signal, thread, msgqueue, etc.
• framework: binder, bluetooth (13), feature (7), media (9), quickapp, security, services (ams/pms), telephony (14), uorb, utils, kvdb
• network: net, net_dhcp, net_ftp, netlib, wapi

Quality Checks

The following automated checks were run before committing:

• File coverage: 69 / 69 (100%)
• API count parity: every Chinese file matches its English counterpart in API count
• Navigation header present in every English file
• No residual Chinese characters (excluding navigation headers)
• No residual Chinese punctuation
• No residual Chinese section titles such as **参数** or **返回值**
• Consistent core terminology (e.g. tapi_context mapped to Telephony context handle, p_handle mapped to Asynchronous callback function)

Testing

Documentation-only change; no runtime testing required. All files pass the local validation scripts.

Stats

• 150 files changed
• +31463 lines / -18 lines