_content/blog/generic-interfaces: new blog post

Change-Id: I1ac78b6a884651d7b9796bd4fa45d9516d91d45e
Reviewed-on: https://go-review.googlesource.com/c/website/+/614595
LUCI-TryBot-Result: Go LUCI <[email protected]>
Reviewed-by: Dmitri Shuralyov <[email protected]>
Reviewed-by: Alan Donovan <[email protected]>

Author: Merovius