_content: Docs - linking update

- updates some links that were pointing to /dl to /doc/install
- Add "About Go Packages" link to the footer nav under "Packages"

Video: https://drive.google.com/file/d/1hAxhpJpHsAnHf5ww267IK6ovWnL-S1dG/view?usp=sharing

Change-Id: I0d71b095a6a60b809a5bceff9cc5fe4a38d5eb91
Reviewed-on: https://go-review.googlesource.com/c/website/+/449415
Reviewed-by: Jamal Carvalho <jamal@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: Jamal Carvalho <jamal@golang.org>
Auto-Submit: Jamal Carvalho <jamal@golang.org>
Reviewed-by: David Chase <drchase@google.com>
diff --git a/_content/index.md b/_content/index.md
index e346227..bcf2581 100644
--- a/_content/index.md
+++ b/_content/index.md
@@ -259,7 +259,7 @@
       </p>
       <div class="GettingStartedGo-ctas">
         <a class="GettingStartedGo-primaryCta" href="/learn/">Get Started</a>
-        <a href="/dl">Download Go</a>
+        <a href="/doc/install/">Download Go</a>
       </div>
     </div>
     <div class="GettingStartedGo-resourcesSection">
diff --git a/_content/menus.yaml b/_content/menus.yaml
index 314fd52..4d83732 100644
--- a/_content/menus.yaml
+++ b/_content/menus.yaml
@@ -93,6 +93,8 @@
     children:
       - name: Standard Library
         url: /pkg/
+      - name: About Go Packages
+        url: https://pkg.go.dev/about
 
   - name: About
     url: /project
diff --git a/_content/tour/concurrency.article b/_content/tour/concurrency.article
index 454768d..ea1613d 100644
--- a/_content/tour/concurrency.article
+++ b/_content/tour/concurrency.article
@@ -171,7 +171,7 @@
 * Where to Go from here...
 
 #appengine: You can get started by
-#appengine: [[/dl/][installing Go]].
+#appengine: [[/doc/install/][installing Go]].
 
 #appengine: Once you have Go installed, the
 The