Document SSL certificate import from official letsencrypt addon in homeassistant

[pieterbos]

https://github.com/Python-roborock/local_roborock_server/blob/main/docs/home_assistant.md#add-on-behavior lists all_addon_configs as a source for certificates. If however you use the official homeassistant let's encrypt addon, they will live in /ssl

I'll likely create a pull request later, documenting this for myself. Hopefully also an onboarding report for a QRevo Edge 2 :)

[Croydon]

As a note, so that nobody else has to figure this out by themselves: /ssl is already mapped into the local roborock server HA app, so referring to certificates being stored in /ssl already works today.