doc: start draft go1.17 release notes, move go1.16 to x/website

This template is based on CL 248198 and previous ones like it.
Continue to eagerly include often-used sections, and clarify that
the TODO is about completing the section, or removing if it turns
out not to be needed.

Move the Go 1.16 release notes to x/website, since that's the new
home for past Go release notes as of CL 291711. They're added to
x/website in CL 295249.

'relnote -html' does not report any CLs with RELNOTE annotations
since 2021/02/01.

For #44513.
Updates #40700.

Change-Id: Idd389335500ec4dec2764cbbaa385918cc8a79ad
Reviewed-on: https://go-review.googlesource.com/c/go/+/295209
Trust: Dmitri Shuralyov <[email protected]>
Run-TryBot: Dmitri Shuralyov <[email protected]>
TryBot-Result: Go Bot <[email protected]>
Reviewed-by: Alexander Rakoczy <[email protected]>
Reviewed-by: Carlos Amedee <[email protected]>