doc: start draft Go 1.20 release notes

doc: start draft Go 1.20 release notes

This initial release notes template is based on previous releases.
CL 425354 adds initial content generated by 'relnote -html'.

For #54202.

Change-Id: I36e15723edc9610d61986ba226ee84c2b30a33af
Reviewed-on: https://go-review.googlesource.com/c/go/+/425334
Reviewed-by: Carlos Amedee <carlos@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Auto-Submit: Dmitri Shuralyov <dmitshur@golang.org>
Run-TryBot: Dmitri Shuralyov <dmitshur@golang.org>
TryBot-Result: Gopher Robot <gobot@golang.org>