function onOpen(e) {

const menu = SpreadsheetApp.getUi().createMenu(APP_TITLE)
  menu
    .addItem('Process invoices', 'processDocuments')
    .addItem('Send emails', 'sendEmails')
    .addSeparator()
    .addItem('Reset template', 'clearTemplateSheet')
    .addToUi();
}
Creates a custom menu in the Google Sheets UI when the document is opened.

@param {object} e The event parameter for a simple onOpen trigger.