- Readme (zh/en): make "download prebuilt SDK, no build" the primary path for
game developers; move build-from-source content under an engine-developers
section.
- Rewrite docs/usr/README.md: download/run, capabilities, debugging,
packaging, and a "things to keep in mind" section.
- Unify wording in MODULES/TRIMMING/dev docs and the SDK README.
- Replace the JS treeview with a plain-HTML static sidebar (home, user guide,
module handbook grouped into 5 categories, API indexes) so docs navigate
without JavaScript.
- Add a Doxygen landing page and page-hierarchy wrappers generated by
docs/CMakeLists.txt; tune Doxyfile for the static layout.
