docs: add coder registry link to docs sidebar (coder#18585)

I am not sure if this works

---------

Co-authored-by: EdwardAngert <17991901+EdwardAngert@users.noreply.github.com>
Co-authored-by: Edward Angert <EdwardAngert@users.noreply.github.com>

Author: matifali

docs/manifest.json

@@ -1767,6 +1767,11 @@
 					]
 				}
 			]
+		},
+		{
+			"title": "Coder Registry",
+			"description": "A collection of Templates and Modules used to create and extend Coder workspaces.",
+			"path": "./registry/index.md"
 		}
 	]
 }

docs/registry/index.md

@@ -0,0 +1,5 @@
+---
+redirect: https://registry.coder.com
+---
+
+If you are not redirected automatically, visit [Coder Registry](https://registry.coder.com).