Add `IMPORTANT` GitHub alert

This will only realistically be displayed within GitHub, so
#linebender > GitHub (markdown) alerts doesn't apply

doc/vision.md

diff --git a/doc/vision.md b/doc/vision.md
index 15f96a6..ee49d7f 100644
--- a/doc/vision.md
+++ b/doc/vision.md
@@ -1,5 +1,6 @@
 # Notice
 
+> [!IMPORTANT]
 > This document represented the vision as of the end of 2020. 
 > Since then a number of things have changed (for example we are now using `wgpu`).
 > For a more up-to-date document, see [the roadmap 2023](./roadmap_2023.md).