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/posts/_index.md

@@ -0,0 +1,6 @@
+---
+title: News
+type: posts
+weight: 10
+geekdocHidden: true
+---

exampleSite/content/en/posts/hello_geekdoc.md

@@ -0,0 +1,13 @@
+---
+title: Hello Geekdoc
+type: posts
+date: 2020-01-06
+tags:
+  - Documentation
+---
+
+This is the first release of the Geekdoc theme.
+
+Dolor sit, sumo unique argument um no. Gracie nominal id xiv. Romanesque acclimates investiture. Ornateness bland it ex enc, est yeti am bongo detract re. Pro ad prompts feud gait, quid exercise emeritus bis e. In pro quints consequent, denim fastidious copious quo ad. Stet probates in duo.
+
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exampleSite/content/en/posts/initial-release.md

@@ -0,0 +1,9 @@
+---
+title: Initial release
+type: posts
+date: 2020-01-08
+---
+
+This is the first release of the Geekdoc theme.
+
+Dolor sit, sumo unique argument um no. Gracie nominal id xiv. Romanesque acclimates investiture. Ornateness bland it ex enc, est yeti am bongo detract re. Pro ad prompts feud gait, quid exercise emeritus bis e. In pro quints consequent, denim fastidious copious quo ad. Stet probates in duo.