fix: translation for navbar menu

docs: update docs for multi-language
chore: add custom css support
chore: add social icons

exampleSite/content/docs/advanced/customization.md

@@ -2,3 +2,7 @@
 title: Customizing Hextra
 linkTitle: Customization
 ---
+
+## Custom CSS
+
+To add custom CSS, we need to create a file `assets/css/custom.css` in our site. Hextra will automatically load this file.

exampleSite/content/docs/advanced/multi-language.md

@@ -0,0 +1,70 @@
+---
+title: "Multi-language"
+weight: 1
+---
+
+Hextra supports creating site with multiple languages using Hugo's [multilingual mode](https://gohugo.io/content-management/multilingual/).
+
+## Enable Multi-language
+
+To make our site multi-language, we need to tell Hugo the supported languages. We need to add to the site configuration file:
+
+```yaml {filename="hugo.yaml"}
+defaultContentLanguage: en
+languages:
+  en:
+    languageName: English
+    weight: 1
+  fr:
+    languageName: Français
+    weight: 2
+  ja:
+    languageName: 日本語
+    weight: 3
+```
+
+## Manage Translations by Filename
+
+Hugo supports managing translations by filename. For example, if we have a file `content/docs/_index.md` in English, we can create a file `content/docs/_index.fr.md` for French translation.
+
+{{< filetree/container >}}
+  {{< filetree/folder name="content" >}}
+    {{< filetree/folder name="docs" state="open" >}}
+      {{< filetree/file name="_index.md" >}}
+      {{< filetree/file name="_index.fr.md" >}}
+      {{< filetree/file name="_index.ja.md" >}}
+    {{< /filetree/folder >}}
+  {{< /filetree/folder >}}
+{{< /filetree/container >}}
+
+Note: Hugo also supports [Translation by content directory](https://gohugo.io/content-management/multilingual/#translation-by-content-directory).
+
+## Translate Menu Items
+
+To translate menu items in the navigation bar, we need to set the `identifier` field:
+
+```yaml {filename="hugo.yaml"}
+menu:
+  main:
+    - identifier: documentation
+      name: Documentation
+      pageRef: /docs
+      weight: 1
+    - identifier: blog
+      name: Blog
+      pageRef: /blog
+      weight: 2
+```
+
+and translate them in the corresponding i18n file:
+
+```yaml {filename="i18n/fr.yaml"}
+documentation: Documentation
+blog: Blog
+```
+
+## Read More
+
+- [Hugo Multilingual Mode](https://gohugo.io/content-management/multilingual/)
+- [Hugo Multilingual Part 1: Content translation](https://www.regisphilibert.com/blog/2018/08/hugo-multilingual-part-1-managing-content-translation/)
+- [Hugo Multilingual Part 2: Strings localization](https://www.regisphilibert.com/blog/2018/08/hugo-multilingual-part-2-i18n-string-localization/)