_content: add footer ref link to pkgsite api

Change-Id: Ib01fac91fddd837a8d7a93265fb372f5e5224066
Reviewed-on: https://go-review.googlesource.com/c/website/+/791960
Reviewed-by: Hyang-Ah Hana Kim <hyangah@gmail.com>
Auto-Submit: Ethan Lee <ethanalee@google.com>
LUCI-TryBot-Result: golang-scoped@luci-project-accounts.iam.gserviceaccount.com <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

Author: ethanalee-work

_content/menus.yaml

@@ -102,6 +102,8 @@ footer:
         url: /pkg/
       - name: About Go Packages
         url: https://pkg.go.dev/about
+      - name: pkg.go.dev API
+        url: https://pkg.go.dev/api
 
   - name: About
     url: /project