doc/initial: initial contents of release notes

Add files to doc/initial that set up the overall structure
of the release notes document.

For #64169.

Change-Id: Ifbf330e554e1fa20d47c72cc309d5cd26048a323
Reviewed-on: https://go-review.googlesource.com/c/go/+/556817
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>

doc/initial/1-intro.md

@@ -0,0 +1,12 @@
+<!--
+NOTE: In this document and others in this directory, the convention is to
+set fixed-width phrases with non-fixed-width spaces, as in
+`hello` `world`.
+-->
+
+<style>
+  main ul li { margin: 0.5em 0; }
+</style>
+
+## Introduction to Go 1.XX {#introduction}
+

doc/initial/2-language.md

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+## Changes to the language {#language}
+
+

doc/initial/3-tools.md

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+## Tools {#tools}
+
+### Go command {#go-command}
+
+### Cgo {#cgo}
+

doc/initial/4-runtime.md

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+## Runtime {#runtime}

doc/initial/5-toolchain.md

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+## Compiler {#compiler}
+
+## Assembler {#assembler}
+
+## Linker {#linker}
+
+

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

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+## Standard library {#library}
+

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

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+### Minor changes to the library {#minor_library_changes}
+
+

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

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+API changes and other small changes to the standard library go here.

doc/initial/7-ports.md

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+## Ports {#ports}
+