library/library_website/views/helloworld_template.xml

<?xml version="1.0"?>
<odoo>
    <template id="helloworlda" name="Hello World Template - a">
        <h1>Hello again World!</h1>
    </template>
    <template id="helloworld" name="Hello World Template">
        <t t-call="website.layout">
            <h1>Hello World!</h1>
        </t>
    </template>
</odoo>