[Idea] Switch translations to a single site

[tesseralis]

Right now each translation has its own subdomain (es.reactjs.org, ja.reactjs.org) run as a separate website. This follows the convention of Vue and Angular but has several disadvantages.

• We lose the advantage of single page apps, since switching languages requires you to load a completely new site.
• Each language site doesn't have access to data from other sites, so we can't do things like only showing languages that have been translated in a dropdown.

Since I'm doing work on doing translations for Gatsby, which the site is based on, I'm thinking of doing some work to consolidate all the different translations under one main site.

This work entails:

• Moving all strings to markdown/yaml in content and removing src/
• Add logic in the main repo to source content from the language repositories and serve them at reactjs.org/es/docs etc.
• Redirect all languages to the main site: e.g. es.reactjs.org to reactjs.org/es