Bring the koreader protocol doc in line with the hash-sharing work:
- Request table: uuid is no longer required (it is absent on the first
sync of a newly downloaded book); document sha256 and file_path and
the resolution priority uuid -> sha256 -> file_path alias ->
title/author
- Note that SHA-256 matching is format-aware (media_items hash first,
media_item_formats fallback) so converted KEPUB/PDF downloads match
- Document the sha256 field returned by the metadata and library
endpoints
- Add a 'Book identification' section pointing current and future
clients (koreader, kobo, OPDS, device-link UI, mobile apps) at the
shared BookResolver as the single resolution path