OPDS: Add standard ebooks feed to existing installs
OPDS: Add standard ebooks feed to existing installs

c.f., https://github.com/koreader/koreader/pull/8606#issuecomment-1008327687

Frenzie/koreader,poire-z/koreader,Frenzie/koreader,koreader/koreader,NiLuJe/koreader,koreader/koreader,poire-z/koreader,NiLuJe/koreader