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.

external/customization-guides/dark-theme.md

@@ -1,7 +1,9 @@
 ---
+lang: en
 layout: doc
 redirect_from:
 - /doc/dark-theme/
 redirect_to: https://github.com/Qubes-Community/Contents/blob/master/docs/customization/dark-theme.md
+ref: 74
 title: Dark Theme in Dom0 and DomU
 ---

external/customization-guides/fedora-minimal-template-customization.md

@@ -1,6 +1,8 @@
 ---
+lang: en
 layout: doc
 redirect_from: /en/doc/fedora-minimal-template-customization/
 redirect_to: https://github.com/Qubes-Community/Contents/blob/master/docs/customization/fedora-minimal-template-customization.md
+ref: 76
 title: Fedora Minimal Template Customization
 ---

external/customization-guides/language-localization.md

@@ -1,4 +1,5 @@
 ---
+lang: en
 layout: doc
 redirect_from:
 - /doc/language-localization/
@@ -6,5 +7,6 @@ redirect_from:
 - /doc/LanguageLocalization/
 - /wiki/LanguageLocalization/
 redirect_to: https://github.com/Qubes-Community/Contents/blob/master/docs/customization/language-localization.md
+ref: 73
 title: Language Localization
 ---

external/customization-guides/removing-templatevm-packages.md

@@ -1,7 +1,9 @@
 ---
+lang: en
 layout: doc
 redirect_from:
 - /doc/removing-templatevm-packages/
 redirect_to: https://github.com/Qubes-Community/Contents/blob/master/docs/customization/removing-templatevm-packages.md
+ref: 75
 title: Removing TemplateVM Packages
 ---

external/customization-guides/windows-template-customization.md

@@ -1,6 +1,9 @@
 ---
+lang: en
 layout: doc
-redirect_from: /en/doc/windows-template-customization/
+redirect_from:
+- /doc/windows-template-customization/
 redirect_to: https://github.com/Qubes-Community/Contents/blob/master/docs/customization/windows-template-customization.md
+ref: 72
 title: Windows Template Customization
 ---