Add missing 404 response to Swagger (#27038)

Most middleware throw a 404 in case something is not found e.g. a Repo
that is not existing. But most API endpoints don't include the 404
response in their documentation. This PR changes this.

routers/api/v1/repo/blob.go

@@ -38,6 +38,8 @@ func GetBlob(ctx *context.APIContext) {
 	//     "$ref": "#/responses/GitBlobResponse"
 	//   "400":
 	//     "$ref": "#/responses/error"
+	//   "404":
+	//     "$ref": "#/responses/notFound"
 
 	sha := ctx.Params("sha")
 	if len(sha) == 0 {