Local rendering: Missing Icon for submenus

[ineswillenbrock]

When rendering documentation locally, there won't be any icons indicating submenus:

It should look like this:

Missing:

<span class="toctree-expand" tabindex="0"></span>

[ineswillenbrock]

Suspicious:

• the workaround for mainmenu.json yields CORS Missing Allow Origin in Firefox
• there are no(?) events from packages/typo3-docs-theme/assets/js/menu-expandable.js