docs: add elysia-background to community plugins list (#558)

Author: staciax

docs/plugins/overview.md

@@ -116,6 +116,7 @@ This is to ensure developers end up with a performant web server they intend to
 -   [Elysia Prometheus](https://github.com/m1handr/elysia-prometheus) - Elysia plugin for exposing HTTP metrics for Prometheus.
 -   [Elysia Remote DTS](https://github.com/rayriffy/elysia-remote-dts) - A plugin that provide .d.ts types remotely for Eden Treaty to consume.
 -   [Cap Checkpoint plugin for Elysia](https://capjs.js.org/guide/middleware/elysia.html) - Cloudflare-like middleware for Cap, a lightweight, modern open-source CAPTCHA alternative designed using SHA-256 PoW.
+-   [Elysia Background](https://github.com/staciax/elysia-background) - A background task processing plugin for Elysia.js
 
 ## Complementary projects: