doc: 2021 is the Year of the Gopher

What a year it has been. If there's one thing we can count on
at a time like this, it's that 2021 is the Year of the Gopher.

Change-Id: I4f6fbbe3d9b4b70df8fadd0d18237f05cd572265
Reviewed-on: https://go-review.googlesource.com/c/go/+/280163


Trust: Dmitri Shuralyov <dmitshur@golang.org>
Run-TryBot: Dmitri Shuralyov <dmitshur@golang.org>
TryBot-Result: Go Bot <gobot@golang.org>
Reviewed-by: Alberto Donizetti <alb.donizetti@gmail.com>
Reviewed-by: Brad Fitzpatrick <bradfitz@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>