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/docs/api/plugins/plugin-content-pages.md

@@ -66,3 +66,24 @@ module.exports = {
   ],
 };
 ```
+
+## i18n
+
+Read the [i18n introduction](../../i18n/i18n-introduction.md) first.
+
+### Translation files location
+
+- **Base path**: `website/i18n/<locale>/docusaurus-plugin-content-pages`
+- **Multi-instance path**: `website/i18n/<locale>/docusaurus-plugin-content-pages-<pluginId>`
+- **JSON files**: extracted with [`docusaurus write-translations`](../../cli.md#docusaurus-write-translations)
+- **Markdown files**: `website/i18n/<locale>/docusaurus-plugin-content-pages`
+
+### Example file-system structure
+
+```bash
+website/i18n/<locale>/docusaurus-plugin-content-pages
+│
+│ # translations for website/src/pages
+├── first-markdown-page.md
+└── second-markdown-page.md
+```