feat: add multilingual mode support (#241)

BREAKING CHANGE:  Layout file renamed ´layouts/partials/page-footer.html → layouts/partials/menu-nextprev.html`. If you use overrides, you might need to change the filenames as well.

exampleSite/content/en/features/multilingual/_index.md

@@ -0,0 +1,78 @@
+---
+title: Multilingual
+resources:
+  - name: translation-available
+    src: images/translation-available.png
+    title: ""
+---
+
+{{< toc >}}
+
+Hugo supports the creation of websites with multiple languages. In this post we will explain how to get configure Multilingual Mode with this theme.
+
+## Configuration
+
+### Languages
+
+You need to set a default language and configure at least two different languages used by your site to your configuration file at `config.toml`:
+
+```Toml
+defaultContentLanguage = "en"
+
+[languages.en]
+languageName = "English"
+contentDir = "content/en"
+weight = 10
+
+[languages.de]
+languageName = "German"
+contentDir = "content/de"
+weight = 20
+```
+
+### Translation Strings
+
+To customize translation strings used by the theme you can create a file `i18n/<languagecode>.toml` for every language you want to use e.g. `i18n/en.toml`. You can lookup all used strings in the [default](https://github.com/thegeeklab/hugo-geekdoc/blob/main/i18n/en.yaml) translation file.
+
+### Menus
+
+For the [Bundle Menu](/usage/menus/#bundle-menu) as well as for the [Extra Header Menu](/usage/menus/#extra-header-menu) you can translate the name within the data file of the menu:
+
+```YAML
+---
+more:
+  # If `name` is a text, this text will be used as name for each language.
+  - name: News
+    ref: "/#"
+    icon: "gdoc_notification"
+  # To translate the name you can add a sub-item per language. Important: If you miss a language key
+  # that is configured in the languages list of your `config.toml` the name will be empty for this language!
+  - name:
+      en: Releases
+      de: Veröffentlichung
+    ref: "https://github.com/thegeeklab/hugo-geekdoc/releases"
+    external: true
+    icon: "gdoc_download"
+```
+
+## Add Content
+
+To translate your content you need to create a directory `content/<languagecode>/` for each language you want to use e.g. `content/en/`. This language directories will hold the translated pages for the particular language.
+
+## Switch Content
+
+If you have configured at least two different languages, the language switcher will be enabled in the UI automatically. The switcher is as part of the header menu and displayed on all pages.
+
+{{< columns >}}
+
+[![Beach Color Palette](images/translation-available.png)](images/translation-available.png)
+
+On pages for which a translation is available it will be displayed in the selection list and links to the translated page.
+
+<--->
+
+[![Beach Color Palette](images/translation-not-available.png)](images/translation-not-available.png)
+
+Pages without a translation will be displayed in the selection list as well but are marked with an asterisk and link to the start page of the respective language.
+
+{{< /columns >}}