Move the English root site to mdBook, keep the Chinese site as a sub-book, and update CI/deploy to publish .mdbook outputs to docs/ and docs/cn/. Also add regression coverage for placeholder skipping, publish-tree assembly, and shared resource setup.
Add mdBook configuration rooted at zh_chapters, generate and commit SUMMARY.md, rewrite d2l-specific directives through a Python preprocessor, refresh chapter resource symlinks from the build scripts, and ignore local build-only links and helper directories.
