feat: add playpen topic to meta

Author: jsjoeio

src/meta.md

@@ -6,5 +6,7 @@ everyone. These topics include:
 
 - [Documentation][doc]: Generate library documentation for users via the included
   `rustdoc`.
+- [Playpen][playpen]: Integrate the Rust Playpen(also known as the Rust Playground) in your documentation.
 
 [doc]: meta/doc.md
+[playpen]: meta/playpen.md