docs: update note formatting (#1893)

THe way Github parse markdown to generate admonition have been changed
as explained in https://github.com/orgs/community/discussions/16925. The
trick used by @choldgraf was not working anymore.

README.md

@@ -37,7 +37,7 @@ and use as follows:
 
 And that's it!
 
-> **Note**
+> [!NOTE]
 > This theme may not work with the latest major versions of Sphinx, especially
 > if they have only recently been released. Please give us a few months of
 > time to work out any bugs and changes when new releases are made.