Completely replace the opener, which had become not only stale
but bad, expand the discussion of the gopher, and generally provide
prose more connected to the present than to the programming world
of 2007.

Fixes #26107

Change-Id: I5e72f0c81e71d1237fe142dc26114991329a6996
Reviewed-on: https://go-review.googlesource.com/124616Reviewed-by: Ian Lance Taylor <iant@golang.org>