📚 Move composing-myst-yml from note to section in docs

docs/frontmatter.md

@@ -55,7 +55,7 @@ project:
 ```
 
 (composing-myst-yml)=
-:::{note} Composing multiple `.yml` files
+#### Composing multiple `.yml` files
 
 You may separate your frontmatter into multiple, composable files. To reference other files from your main `myst.yml` file, use the `extends` key with relative path(s) to the other configuration files:
 
@@ -71,7 +71,6 @@ extends:
 Each of these files listed under `extends` must contain valid `myst.yml` structure with `version: 1` and `site` or `project` keys. They may also have additional files listed under `extends`.
 
 Composing files together this way allows you to have a single source of truth for project frontmatter that may be reused across multiple projects, for example math macros or funding information.
-:::
 
 +++