v0.20.1: Release 0.20.1: docs links in check reports, too_fast at scene cuts.

Added

• Every issue in report.json has docs: the link to its rule on
  https://oxframe.dev/docs/check-rules, with what the rule reports and how to
  fix it (Issue::docs).
• The showreel example: scenes from other examples between a title and an
  end card, the reel on oxframe.dev.

Fixed

• check reported too_fast at a cut between two scenes when the first
  node of each had the same id: node ids start again in every scene, and
  a move is now only a node added at the same place.