doc/go1.15: create go1.15.html

Copied from go1.14.html, with changes redacted back to TODOs following
the model of CL 195058.

'relnote -html' does not report any changes at this time.

Updates #33738

Change-Id: I580232805ab7db35935f3e1ba03b720be4796a7e
Reviewed-on: https://go-review.googlesource.com/c/go/+/220278


Reviewed-by: Alexander Rakoczy <alex@golang.org>
Run-TryBot: Alexander Rakoczy <alex@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>