Uninstall guide missing delete storage part

[pradel]

Bug report

Describe the bug

We need to update the uninstall guide and add the missing part.

Side note: I think the uninstall instructions are missing cleaning up the storage. I followed them to uninstall (destroying the app and databases) and then when running the bootstrap script it failed to create storage because it was already there (/var/lib/dokku/data/storage/ledokku-ssh).

https://discord.com/channels/781833375236489216/820245592679776287/839148628764393533