Docs: Fix language switching for examples.

[Mugen87]

Fixed #16669

[Mugen87]

https://raw.githack.com/Mugen87/three.js/dev32/docs/index.html

[mrdoob]

So the problem is that https://github.com/mrdoob/three.js/tree/dev/docs/examples doesn't have en and zh folders, right?

[Mugen87]

Yes, that's right. Do you think it's better to re-organize the file structure of docs/examples instead of changing the JS code?

[Mugen87]

Okay, I've introduced the same file structure (en/zh) similar to the api and manual directories. In this way, the doc pages are now ready for Chinese translation, too.

[mrdoob]

Awesome! Many thanks!