content: Add link to Czech translation

content/welcome.article

@@ -51,6 +51,7 @@ The tour is available in other languages:
 - [[https://go-tour-ca.appspot.com/][Catalan — Català]]
 - [[https://tour.go-zh.org/][Simplified Chinese — 中文（简体）]]
 - [[https://go-tour-zh-tw.appspot.com/][Traditional Chinese — 中文（繁體）]]
+- [[https://go-tour-cz.appspot.com/][Czech — Česky]]
 - [[https://go-tour-fr.appspot.com/][French — Français]]
 - [[https://go-tour-de.appspot.com/][German — Deutsch]]
 - [[https://go-tour-he.appspot.com/][Hebrew — עִבְרִית]]