Restructure README and move technical guides into docs

[xymeow]

What changed

• move How to use / 如何使用 directly after the real-device demo
• reduce both root READMEs to under 200 lines
• move version-bound installation, configuration, API, and development details into paired English/Chinese guides under docs/
• make host/README* point to the canonical getting-started guides
• update the commentary-style documentation contract to validate the canonical Device API guides
• add explicit documentation boundaries for human maintainers and AI agents

Why

The root README had grown to roughly 500 lines, placing usage behind a long installation section and duplicating version-sensitive commands. This keeps the project landing page task-oriented while preserving exact commands and contracts in versioned repository documentation.

GitHub Wiki content is prepared separately for navigation, FAQ, and field debugging. It will be published after this PR reaches main so its canonical blob/main links do not return 404.

User and developer impact

• new users reach the operating flow before architecture and installation details
• maintainers and agents get paired, versioned guides with explicit source-of-truth boundaries
• host build instructions remain canonical in host/README*
• no watcher, mod, firmware, TTS, device behavior, or configuration defaults change

Validation

• python3 -m unittest discover -s tools -p 'test_*.py' — 167 passed
• node tools/test_stackchan_mod_web_behavior.mjs — passed
• all local Markdown/image links resolve
• git diff --check — passed
• verified the official Stack-chan Web Console replacement URL returns HTTP 200