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

packages/docusaurus-init/templates/bootstrap/docs/interactiveDoc.mdx

@@ -0,0 +1,27 @@
+---
+id: mdx
+title: Powered by MDX
+---
+
+You can write JSX and use React components within your Markdown thanks to [MDX](https://mdxjs.com/).
+
+The `.mdx` extension is not required, but will enable better support from tooling (IDE, Prettier...).
+
+export const Highlight = ({children, color}) => (
+  <span
+    style={{
+      backgroundColor: color,
+      borderRadius: '2px',
+      color: '#fff',
+      padding: '0.2rem',
+    }}
+    onClick={() => alert('Highlight pressed!')}>
+    {children}
+  </span>
+);
+
+<Highlight color="#25c2a0">Docusaurus green</Highlight> and <Highlight color="#1877F2">
+  Facebook blue
+</Highlight> are my favorite colors.
+
+I can write **Markdown** alongside my _JSX_!