chore: update README.md (admonitions) (#3712)

* Place admonitions in code blocks

* Add missing newline

---------

Co-authored-by: Karl Cardenas <29551334+karl-cardenas-coding@users.noreply.github.com>

README.md

@@ -472,41 +472,53 @@ Result
 
 ### Admonitions - Warning / Info / Tip / Danger / Tech Preview / Further Guidance
 
+```mdx
 :::warning
 
 Some **content** with _Markdown_ `syntax`.
 
 :::
+```
 
+```mdx
 :::info
 
 Some **content** with _Markdown_ `syntax`.
 
 :::
+```
 
+```mdx
 :::tip
 
 Some **content** with _Markdown_ `syntax`.
 
 :::
+```
 
+```mdx
 :::danger
 
 Some **content** with _Markdown_ `syntax`.
 
 :::
+```
 
+```mdx
 :::preview
 
 Some **content** with _Markdown_ `syntax`.
 
 :::
+```
 
+```mdx
 :::further
 
 Some **content** with _Markdown_ `syntax`.
 
 :::
+```
 
 https://docusaurus.io/docs/markdown-features/admonitions