_content/blog/go-brand: link to go.dev/brand

More formal brand guidelines are now available.

For golang/go#58412.

Change-Id: If3383c186115f146577ad80fddb7e1c5744af157
Reviewed-on: https://go-review.googlesource.com/c/website/+/558195
Auto-Submit: Russ Cox <rsc@golang.org>
Reviewed-by: Sameer Ajmani <sameer@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

_content/blog/go-brand.md

@@ -87,6 +87,9 @@ The brand guide contains the guidelines for acceptable logo use.
   - [Go Logos (zip)](/s/logos)
   - [Go Slide Masters (Google Slides)](/s/presentation-theme)
 
+_Update, June 2023_:
+The guidelines for acceptable logo use are now at [go.dev/brand](/brand).
+
 ## What’s happening next
 
 The website will be getting a refresh based on the new design. Since we are