Reorganize the legacy docs/doxygen/ directory so that file locations
match both the actual content type and the existing bilingual docs
layout (zh-cn/ for Chinese, en/ for English).
== Why ==
Since #526 deprecated the Sphinx/Doxygen build pipeline, all files
under docs/doxygen/ are plain hand-written Markdown. The directory
name is misleading, and the content is currently Chinese-only while
the rest of docs/ uses docs/zh-cn and docs/en mirrors.
The directory also mixed two unrelated content types:
- docs/doxygen/api/ — API reference
- docs/doxygen/system_debug_tools/ — Chinese developer guides
== What ==
- git mv docs/doxygen/api → docs/zh-cn/api (81 files)
- git mv docs/doxygen/system_debug_tools → docs/zh-cn/device_dev_guide/system_debug_tools (20 files)
- rmdir docs/doxygen (now empty)
Pure rename / move; no content changes. 'grep -rn doxygen/' over the
remaining Markdown and YAML returns no hits.
== Notes ==
- API documentation is now at docs/zh-cn/api/, consistent with the rest
of the Chinese docs tree. The English mirror at docs/en/api/ will be
added in a follow-up PR once translations are ready.
- system_debug_tools/ joins the existing Chinese dev-guide tree
alongside kernel/, driver/, connection/, media/, etc.
- The system_debug_tools/ files are currently wrapped in Doxygen-style
comment blocks; that formatting issue is tracked as a separate
cleanup task and is not addressed here.
zhangxiaowei16