doc: add alt attribute to gopher image in help.html

This adds an alt tag for accessibility. The alt text is a visual
description of the text that is read out loud to users using a
screen reader. The HTML specifications indicate that alt tags for
decorative images should be left blank.

Fixes #39861

Change-Id: I76c39a461ceabe685826aa46e4f26ad893d50634
Reviewed-on: https://go-review.googlesource.com/c/go/+/240258


Reviewed-by: Alexander Nohe <alex.nohe427@gmail.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>