Add `artefacts/paperdown.schema.json`

[atsyplenkov]

See https://github.com/eitsupi/arf/tree/main/artifacts and https://github.com/etiennebacher/jarl/tree/main/artifacts

[atsyplenkov]

Split config into two categories:

[[general]]
verbose = false
overwrite = false
normalize-tables = false

[[auth]]
env-file = ".env"

[[connection]]
timeout = 180
max-download-bytes = 20971520
workers = 32
ocr-workers = 2