content/static/html/pages: link to package-adding instructions on 404

In addition to providing a frontend fetch button, link to the manual
way to add a package.

Fixes golang/go#41347

Change-Id: Iaf767e0cff773ddb9f099067d6bc11e2d6fd25e7
Reviewed-on: https://go-review.googlesource.com/c/pkgsite/+/255097
Trust: Jonathan Amsterdam <jba@google.com>
Run-TryBot: Jonathan Amsterdam <jba@google.com>
TryBot-Result: kokoro <noreply+kokoro@google.com>
Reviewed-by: Julie Qiu <julie@golang.org>
diff --git a/content/static/html/pages/fetch.tmpl b/content/static/html/pages/fetch.tmpl
index fb8817b..cca5302 100644
--- a/content/static/html/pages/fetch.tmpl
+++ b/content/static/html/pages/fetch.tmpl
@@ -18,7 +18,9 @@
         <i class="Fetch-dot"></i>
       </div>
       <p class="Fetch-messageSecondary js-fetchMessageSecondary" aria-live="polite">
-        Check that you entered the URL correctly, or request to add “{{.MessageData}}” to pkg.go.dev.
+        Check that you entered the URL correctly,
+	try fetching it following the <a href="/about#adding-a-package">instructions here</a>,
+	or request to add “{{.MessageData}}” to pkg.go.dev.
       </p>
       <button class="Fetch-button js-fetchButton" aria-live="polite">Request “{{.MessageData}}”</button>
     </div>