_content/tour: add a link to the Ukraininan translation

Add a link to the Ukrainian translation to "A Tour of Go".

Link: https://go-tour-ua-translation.lm.r.appspot.com

Fixes golang/go#59945.

Change-Id: I688c6d89914e8bcce4b71a9eb381fe3c196776bd
GitHub-Last-Rev: 1d5608cb80
GitHub-Pull-Request: golang/website#224
Reviewed-on: https://go-review.googlesource.com/c/website/+/492215
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Auto-Submit: Dmitri Shuralyov <dmitshur@google.com>
Run-TryBot: Dmitri Shuralyov <dmitshur@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Jamal Carvalho <jamal@golang.org>

_content/tour/welcome.article

@@ -56,6 +56,7 @@ The tour is available in other languages:
 - [[https://go-tour-ko.appspot.com/][Korean — 한국어]]
 - [[https://go-tour-pl1.appspot.com/][Polish — Polski]]
 - [[https://go-tour-th.appspot.com/][Thai — ภาษาไทย]]
+- [[https://go-tour-ua-translation.lm.r.appspot.com/][Ukrainian — Українською]]
 
 Click the [[javascript:highlightAndClick(".next-page")]["next"]] button or type `PageDown` to continue.