📘docs: add etag plugin

Author: bogeychan

docs/plugins/overview.md

@@ -55,6 +55,7 @@ That's why Elysia is creating pre-built common pattern plugin for convinient usa
 - [Autoroutes](https://github.com/wobsoriano/elysia-autoroutes) - file system routes for Elysia
 - [Group Router](https://github.com/itsyoboieltr/elysia-group-router) - file system and folder-based router for groups.
 - [Basic Auth](https://github.com/itsyoboieltr/elysia-basic-auth) - basic http authentication for Elysia.
+- [ETag](https://github.com/bogeychan/elysia-etag) - automatic HTTP ETags generation for Elysia.
 
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 If you have plugin written for Elysia, feels free to share you plugin by creating PR to [documentation repo](https://github.com/elysiajs/documentation).