Move the homepage language switch below the GitHub star button and restyle it to match the same button family on both the English and Chinese homepages.
Add an English-specific frontpage template so the default homepage no longer falls back to the Chinese frontpage, and clear homepage image backgrounds in the frontpage wrapper CSS.
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.
