_content/doc: add merged go1.23.md

Using doc/next content as of 2024-06-21 (commit 1b4f1dc95d221c1e9d0afb9067fd6a09f12dd061).

For golang/go#65614.

Change-Id: Ifa21855a92dc99f480ea605947cc48d542ff5193
Reviewed-on: https://go-review.googlesource.com/c/website/+/594035
Auto-Submit: Gopher Robot <gobot@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: David Chase <drchase@google.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>