Adds a feature page covering the rolling window, what counts, the MB/GB
estimate and the Docker/CLI keys, plus a one-line pointer from getting started
where a new user with a large Plus library clicks Begin Book Backups.
Scripts/seed-download-history.cs seeds fake completed downloads so the limit
can be exercised without downloading, including dating rows just under 24 hours
old to turn the multi-day pause-and-resume behavior into a one minute test.
Also shortens the queue status text: the process queue column clips rather than
wrapping, so the resume time was being cut off.
Co-authored-by: rmcrackan <rmcrackan@gmail.com>
