doc: initialize next directory for Go 1.25

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

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

For #70525

Change-Id: I31c271d95ccd72123f531fde4e72929ec7b310d9
Reviewed-on: https://go-review.googlesource.com/c/go/+/646195
Reviewed-by: Carlos Amedee <carlos@golang.org>
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>

doc/next/1-intro.md

@@ -0,0 +1,8 @@
+<style>
+  main ul li { margin: 0.5em 0; }
+</style>
+
+## DRAFT RELEASE NOTES — Introduction to Go 1.N {#introduction}
+
+**Go 1.25 is not yet released. These are work-in-progress release notes.
+Go 1.25 is expected to be released in August 2025.**

doc/next/2-language.md

@@ -0,0 +1,3 @@
+## Changes to the language {#language}
+
+

doc/next/3-tools.md

@@ -0,0 +1,6 @@
+## Tools {#tools}
+
+### Go command {#go-command}
+
+### Cgo {#cgo}
+

doc/next/4-runtime.md

@@ -0,0 +1 @@
+## Runtime {#runtime}

doc/next/5-toolchain.md

@@ -0,0 +1,7 @@
+## Compiler {#compiler}
+
+## Assembler {#assembler}
+
+## Linker {#linker}
+
+

doc/next/6-stdlib/0-heading.md

@@ -0,0 +1,2 @@
+## Standard library {#library}
+

doc/next/6-stdlib/99-minor/0-heading.md

@@ -0,0 +1,3 @@
+### Minor changes to the library {#minor_library_changes}
+
+

doc/next/6-stdlib/99-minor/README

@@ -0,0 +1 @@
+API changes and other small changes to the standard library go here.

doc/next/7-ports.md

@@ -0,0 +1,2 @@
+## Ports {#ports}
+