minor: Set the experimental AsciiDoc document attribute to generated draft changelog

The keyboard macro (`kbd:[...]`) requires this attribute.

This default change will prevent issues like
https://github.com/rust-analyzer/rust-analyzer.github.io/pull/191 .

xtask/src/release/changelog.rs

@@ -65,6 +65,7 @@ pub(crate) fn get_changelog(
         "\
 = Changelog #{}
 :sectanchors:
+:experimental:
 :page-layout: post
 
 Commit: commit:{}[] +