Add lang + ref tags to frontmatter

Those are fields used by the language switcher to correlate pages across
different languages, even if they have different names/paths/titles.
They are generated with the prepare_for_translation.py script.

developer/general/devel-books.md

@@ -1,10 +1,12 @@
 ---
+lang: en
 layout: doc
 permalink: /doc/devel-books/
 redirect_from:
 - /en/doc/devel-books/
 - /doc/DevelBooks/
 - /wiki/DevelBooks/
+ref: 32
 title: Developer Books
 ---
 

developer/general/doc-guidelines.md

@@ -1,10 +1,12 @@
 ---
+lang: en
 layout: doc
 permalink: /doc/doc-guidelines/
 redirect_from:
 - /en/doc/doc-guidelines/
 - /wiki/DocStyle/
 - /doc/DocStyle/
+ref: 30
 title: Documentation Guidelines
 ---
 

developer/general/gsoc.md

@@ -1,7 +1,9 @@
 ---
+lang: en
 layout: sidebar
 permalink: /gsoc/
 redirect_from: /GSoC/
+ref: 33
 title: Google Summer of Code
 ---
 

developer/general/gsod.md

@@ -1,6 +1,8 @@
 ---
+lang: en
 layout: sidebar
 permalink: /gsod/
+ref: 28
 title: Google Season of Docs
 ---
 

developer/general/join.md

@@ -1,6 +1,8 @@
 ---
+lang: en
 layout: sidebar
 permalink: /join/
+ref: 26
 title: Join
 ---
 

developer/general/package-contributions.md

@@ -1,6 +1,8 @@
 ---
+lang: en
 layout: doc
 permalink: /doc/package-contributions/
+ref: 29
 title: Package Contributions
 ---
 

developer/general/style-guide.md

@@ -1,5 +1,7 @@
 ---
+lang: en
 layout: doc
 permalink: /doc/style-guide/
+ref: 27
 title: Style-guide
 ---

developer/general/usability-ux.md

@@ -1,6 +1,8 @@
 ---
+lang: en
 layout: doc
 permalink: /doc/usability-ux/
+ref: 31
 title: Usability & UX
 ---