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 <ab5e2bca84933118bbc9d48ffaccce3bac4eeb64@golang.org>
Reviewed-by: Dmitri Shuralyov <d181b7fea0ec87c86ba5a890ab716db52498e3ba@google.com>
Auto-Submit: Dmitri Shuralyov <d181b7fea0ec87c86ba5a890ab716db52498e3ba@golang.org>
Run-TryBot: Dmitri Shuralyov <d181b7fea0ec87c86ba5a890ab716db52498e3ba@golang.org>
TryBot-Result: Gopher Robot <66cb808b70d30c07676d5e946fee83fd561249e5@golang.org>

golang/go,golang/go,golang/go,golang/go,golang/go,golang/go,golang/go,golang/go