tour: add a Spanish translation

I'm part of the Gophers-Latam Community. I am excited to present our
translation of the Tour of Go, a collaborative effort of our community.
It's available at https://go-tour-lat.appspot.com/.

Fixes golang/go#64454

Change-Id: Icd6e1f8ec340deea992858571c03bc2ae933d72b
GitHub-Last-Rev: 45234eb657
GitHub-Pull-Request: golang/website#264
Reviewed-on: https://go-review.googlesource.com/c/website/+/545855
Reviewed-by: Thiago Mowszet <thiagomowszet@gmail.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Auto-Submit: Dmitri Shuralyov <dmitshur@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Carlos Amedee <carlos@golang.org>

_content/tour/welcome.article

@@ -55,6 +55,7 @@ The tour is available in other languages:
 - [[https://go-tour-jp.appspot.com/][Japanese — 日本語]]
 - [[https://go-tour-ko.appspot.com/][Korean — 한국어]]
 - [[https://go-tour-pl1.appspot.com/][Polish — Polski]]
+- [[https://go-tour-lat.appspot.com/][Spanish — Español]]
 - [[https://go-tour-th.appspot.com/][Thai — ภาษาไทย]]
 - [[https://go-tour-ua-translation.lm.r.appspot.com/][Ukrainian — Українською]]