In Redactor, you can insert pre-made pieces of content into your content. You can do this with the Snippets plugin. See its page for a detailed description of how it works.
Click the 'Add' (+) button on the toolbar to open addbar and then click on 'Snippets'.
<!DOCTYPE html>
<html>
<head>
<title>Redactor</title>
<meta charset="utf-8">
<!-- css -->
<link rel="stylesheet" href="/your-folder/redactor.css" />
</head>
<body>
<!-- element -->
<textarea id="entry">...</textarea>
<!-- js -->
<script src="/your-folder/redactor.js"></script>
<script src="/your-folder/plugins/snippets/snippets.js"></script>
<!-- call -->
<script>
Redactor('#entry', {
plugins: ['snippets'],
snippets: '/your-folder/snippets.json'
});
</script>
</body>
</html>