Currently including `zh_CN` and `es` which both have very high completion
ratios. Others will be added once they reach a significant percentage too.
These RST files will be used by godot-docs in place of its `classes` folder
after we sync with https://github.com/godotengine/godot-docs/pull/5458.
The update workflow is manual for now (example for `zh_CN`):
- Build `godotengine/godot` in the branch we currently track (now `3.x`)
- Run `godot --doctool -l zh_CN`
- Run `cd doc && make rst LANGARG=zh_CN`
- Copy `doc/_build/rst/*` to `classes/zh_CN/` here
- Make sure to have `classes/zh_CN/index.rst` copied from `docs/classes`
