_content/doc: add Go 1.17 release notes

Copied from Go commit 20a620fd9f7bc35739c1af3602d53808d0430814.
deleted there in CL 342070.

Updates golang/go#44513

Change-Id: I435253feb47b222423a453c51d0c1ced30e86633
Reviewed-on: https://go-review.googlesource.com/c/website/+/342089
Trust: Carlos Amedee <[email protected]>
Run-TryBot: Carlos Amedee <[email protected]>
TryBot-Result: Go Bot <[email protected]>
Reviewed-by: Dmitri Shuralyov <[email protected]>