_content/doc: add go1.26.md placeholder

Add this file now that it's possible to preview the release notes draft
at https://tip.golang.org/doc/go1.26.

For golang/go#75005.

Change-Id: I9189840edac997f4bf6a338dad15640e4e056b0f
Reviewed-on: https://go-review.googlesource.com/c/website/+/695995
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Auto-Submit: Dmitri Shuralyov <dmitshur@google.com>
Reviewed-by: David Chase <drchase@google.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>

Author: dmitshur

_content/doc/go1.26.md

@@ -0,0 +1,13 @@
+---
+title: Go 1.26 Release Notes
+template: true
+---
+
+{{/*
+	This is a placeholder template that makes it possible to preview
+	Go 1.26 release notes draft at https://tip.golang.org/doc/go1.26.
+	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}}