Audit i18n SEO issues: hreflang, x-default, canonical...

### Have you read the Contributing Guidelines on issues?

- [X] I have read the [Contributing Guidelines on issues](https://github.com/facebook/docusaurus/blob/main/CONTRIBUTING.md#reporting-new-issues).

### Motivation

It has been reported that we may not implement correctly meta page headers for SEO

https://github.com/facebook/docusaurus/issues/3317#issuecomment-975150571
https://github.com/facebook/docusaurus/issues/3317#issuecomment-989136495

Let's discuss here what we do wrong exactly, and what we should do instead.

Please link to reference material from an SEO authoritative source (Google, Moz...) whenever possible.
It's not always so easy to interpret these docs correctly.

Please give concrete examples of what we do wrong and how to solve the problem.
- Page link: ...
- Current wrong meta headers: ...
- Expected (good) meta headers: ...
- Ref link: ...

cc @mxhdx @thadguidry

### Self-service

- [ ] I'd be willing to do some initial work on this proposal myself.