doc: start draft Go 1.21 release notes

This initial release notes template is based on previous releases.

For #58645.

Change-Id: I8bae25d6dd9778a69963b8b4903a6dc1c180e15f
Reviewed-on: https://go-review.googlesource.com/c/go/+/470296
Run-TryBot: Than McIntosh <thanm@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@golang.org>
Reviewed-by: Dmitri Shuralyov <dmitshur@google.com>

doc/go1.21.html

@@ -0,0 +1,77 @@
+<!--{
+	"Title": "Go 1.21 Release Notes",
+	"Path":  "/doc/go1.21"
+}-->
+
+<!--
+NOTE: In this document and others in this directory, the convention is to
+set fixed-width phrases with non-fixed-width spaces, as in
+<code>hello</code> <code>world</code>.
+Do not send CLs removing the interior tags from such phrases.
+-->
+
+<style>
+  main ul li { margin: 0.5em 0; }
+</style>
+
+<h2 id="introduction">DRAFT RELEASE NOTES — Introduction to Go 1.21</h2>
+
+<p>
+  <strong>
+    Go 1.21 is not yet released. These are work-in-progress
+    release notes. Go 1.21 is expected to be released in August 2023.
+  </strong>
+</p>
+
+<h2 id="language">Changes to the language</h2>
+
+<p>
+  TODO: complete this section
+</p>
+
+<h2 id="ports">Ports</h2>
+
+<p>
+  TODO: complete this section, or delete if not needed
+</p>
+
+<h2 id="tools">Tools</h2>
+
+<h3 id="go-command">Go command</h3>
+
+<p>
+  TODO: complete this section, or delete if not needed
+</p>
+
+<h2 id="runtime">Runtime</h2>
+
+<p>
+  TODO: complete this section, or delete if not needed
+</p>
+
+<h2 id="compiler">Compiler</h2>
+
+<p>
+  TODO: complete this section, or delete if not needed
+</p>
+
+<h2 id="linker">Linker</h2>
+
+<p>
+  TODO: complete this section, or delete if not needed
+</p>
+
+<h2 id="library">Core library</h2>
+
+<h3 id="minor_library_changes">Minor changes to the library</h3>
+
+<p>
+  As always, there are various minor changes and updates to the library,
+  made with the Go 1 <a href="/doc/go1compat">promise of compatibility</a>
+  in mind.
+  There are also various performance improvements, not enumerated here.
+</p>
+
+<p>
+  TODO: complete this section
+</p>