In Odoo a menu item can link to an external URL. URL action (ir.actions.act_url) allows opening a URL (website/web page) via an Odoo action.
Here is the menu item code and its action:
<record model="ir.actions.act_url" id="act_reports">
<field name="name">Reports</field>
<field name="url">http://www.example.com</field>
<field name="target">new</field>
</record>
<menuitem name="Reports" id="menu_reports" parent="menu_root" action="act_reports"/>