Use the content we already have stored to generate ebook

[Monirzadeh]

In the future, we should try to aim and use the content we already have stored, but that could be complicated and we need to migrate from warc to obelisk (and potentially allow other archival generators) so this is just a general comment and not that you need to perform any action, just so you have that in mind.

Originally posted by @fmartingr in https://github.com/go-shiori/shiori/pull/623#discussion_r1249320362 waiting for #481