_content/doc: add go1.24.md placeholder

Add this file now so that it's possible to preview the latest release
notes draft at https://tip.golang.org/doc/go1.24, and to include that
link in the tracking issue. Based on CL 588057 that did this for 1.23.

For golang/go#68545.

Change-Id: I07f30023d23ec027927f7417ebece4110f221efa
Reviewed-on: https://go-review.googlesource.com/c/website/+/600179
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Cherry Mui <cherryyz@google.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Auto-Submit: Dmitri Shuralyov <dmitshur@golang.org>

_content/doc/go1.24.md

@@ -0,0 +1,13 @@
+---
+title: Go 1.24 Release Notes
+template: true
+---
+
+{{/*
+	This is a placeholder template that makes it possible to preview
+	Go 1.24 release notes draft at https://tip.golang.org/doc/go1.24.
+	Eventually the release note fragments in doc/next will be merged
+	and added here, replacing the "docNext" function call.
+*/}}
+
+{{with docNext}}{{.}}{{else}}No next release note fragments available.{{end}}