Adapt page about content gone for not-subscribed podcasts / Create new page

[keunes]

Short description: Add a note/call-out that if all content of a single podcast is gone, the user probably isn't subscribed and its content being gone.
Or, alternatively, create a new page.

Location: https://antennapod.org/documentation/bugs-first-aid/database-error

Why have this:

To address this:

I read the Bug First Aid “All my podcasts and episodes are gone”
https://forum.antennapod.org/t/downloaded-podcasts-disappear/5881/5?u=keunes

More info:

[ByteHamster]

Not sure we need to add this to the website. We should just release the bug fix