doc: Add LXD UI instructions to Tutorial

[henrycoggillcnc]

I've set myself up with a microcloud using this tutorial, and then used Miona's blog - https://ubuntu.com/blog/lxd_ui - to tell me how to enable the UI for the microcloud LXD. This did work, though maybe it would be useful to add these instructions to the tutorial as well perhaps?

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Document: tutorial/get_started.rst