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

website/docusaurus.config.js

@@ -313,7 +313,16 @@ module.exports = {
             },
           ],
         },
-        // {type: 'localeDropdown', position: 'right'},
+        {
+          type: 'localeDropdown',
+          position: 'right',
+          dropdownItemsAfter: [
+            {
+              to: 'https://github.com/facebook/docusaurus/issues/3526',
+              label: 'Help Us Translate',
+            },
+          ],
+        },
         {
           href: 'https://github.com/facebook/docusaurus',
           position: 'right',