Move "Starting with a prior history" from old wfforward NB to completing_forward_sims.md

[hyanwong]

There is a lot of good material about parameter scaling in https://github.com/tskit-dev/tutorials/blob/main/old-content/docs/wfforward.md#starting-with-a-prior-history. The obvious place for this is in the tutorial about recapitation, at https://github.com/tskit-dev/tutorials/blob/main/completing_forward_sims.md.

See #14 (comment)