doc: initialize next directory for Go 1.24

doc: initialize next directory for Go 1.24

Following the "For the release team" steps in README:

	cd doc
	cp -R initial/ next
	$EDITOR next/1-intro.md

Dropped '*' from the cp -R command to avoid needing to
create an empty next directory with a separate command.

For #68545.
For #67586.

Change-Id: I25215bc35a043e0f26d4a86ffe66eb3c26348fef
Reviewed-on: https://go-review.googlesource.com/c/go/+/600177
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Dmitri Shuralyov <dmitshur@golang.org>
Reviewed-by: Cherry Mui <cherryyz@google.com>