[zh] Add new pages: what-is-opentelemetry and its _index.md

[windsonsea]

• Fix [zh] Add a first docs page #4478
• Adds a new zh page after the zh homepage.

Preview https://deploy-preview-4481--opentelemetry.netlify.app/zh/docs/

[svrnm]

@chalin is there a way to express "go to the language specific docs and if you do not find them go to the english ones"? We will need this more often in the future and eventually it will be hard to track which links are pointing where

[chalin]

Thanks for the first few doc pages @windsonsea.

@svrnm - I still need to experiment with Hugo so see what the best way forward is. We might have a few alternatives with tradeoffs to consider. I'll report back here as soon as I can.

[chalin]

@svrnm - with the config I've setup, the proper thing to do in the zh pages is to lead all URLs with /zh/ unless there is a wish to intentionally target an en page. Does that make sense to you all?

The fallback is implemented via Hugo mounts.

[chalin]

All, this is what the Docs section will look like with an English-page fallback (also see the preview):

...

If you like this, then I propose that we move forward with this approach. I still need to fix the broken links. I'll do that soon, but wanted to get the previous out to you first.

I also need to fix canonical link URLs, but I don't want to hold this PR back because of it.

[chalin]

All checks are passing, and I've implemented all changes I wanted to put in place. This is good to go, and ready for a final approval @svrnm et al.