docs: fix reference sidebar ordering and group the subsystem pages

The VitePress config declared no position for the subsystem or other-interface
sections, so `indexOf` returned -1 and sorted them ahead of every declared
group: the reference landing page's own sidebar entry sat 1549px below the
fold. Four subsystem pages also shared `order` values with pages in the same
section, resolved only by sort stability and array concatenation order.

Section placement and collapse move into the manifest as a per-locale
declaration, and `sectionSpec` throws for an undeclared section instead of
sorting it silently to the top. Subsystem pages are grouped by concern, the
six topical groups collapse until one holds the page being read, and page
order derives from array position.

The projector drops the language-switcher line and repository badge the
canonical pages carry for their GitHub readers. The navigation bar gains the
DeepSeek wordmark, a release-stage tag, and a favicon; the sidebar scrollbar
rests invisible and appears while scrolling. Subsystem pages carry a two-level
outline, and the two plugin-development tracks now cross-link.

docs/cordis-tutorial/index.zh.md

@@ -8,6 +8,8 @@ Cordis 是 DeepSeek Harness SDK 底层的插件框架：它是一个小型运行
 
 如果你想阅读精简的概念参考，而不是逐步实践，请参阅 [Cordis 入门](../cordis-primer.md)。详尽的 API 参考见[子系统页面](../subsystems/core.md)上生成的 `cordis-surface` 区块，以及 [Cordis 核心 API](../cordis-api/context.md)页面。
 
+如果你要为 harness 本身编写插件——由 `cordis.yml` 加载、在 Web UI 中驱动，而不是下面这个启动器——请从[第一个 Harness 插件](../user/develop/basic/index.md)开始。
+
 ## 准备工作
 
 你需要克隆本仓库并安装依赖；[开发指南](../development.md#setup-tutorial)列出了前置条件。本教程不需要 API 密钥；所有示例均可在无密钥环境中运行。