docs(v2): i18n doc + polish (#4014)

* add some initial i18n doc

* i18n doc progress

* i18n tutorial progress

* i18n tutorial progress

* polish Crowdin docs

* i18n sidebar in guides

* polish crowdin doc

* update Crowdin doc a bit

* fix annoying relative link to global site resource in template (breaks i18n tutorial)

* template: use simpler export for homepage

* add markdown page example

* rename mdx.md to interactiveDoc.mdx

* update bootstrap/facebook templates too

* sync init template package scripts

* add slug frontmatter doc

* improve i18n doc

* complete i18n doc

* temporarily enable the localeDropdown

* doc typo

* improve the i18n doc

* Add Git i18n doc

* add missing "--" for npm run options (unfortunately they don't get stripped by npm2yarn, and are required foor npm)

* improve a bit the Crowdin doc

src/pages/index.js

@@ -54,7 +54,7 @@ function Feature({imageUrl, title, description}) {
  );
 }
 
-function Home() {
+export default function Home() {
  const context = useDocusaurusContext();
  const {siteConfig = {}} = context;
  return (
@@ -93,5 +93,3 @@ function Home() {
  </Layout>
  );
 }
-
-export default Home;