content/sample-episode-rst: Fix 'pending_xref' error with links in exercise list

[rkdarst]

• If we make a copy of the nodes we re-insert (exercise_node.copy()), the error goes away.
• Interestingly, the error stays when you use either exercise_node.deepcopy() or copy.deepcopy(exercise_node).
• First pushing without the fix to ensure CI sees the error, then will force-push with the fix.
• Closes: #83
• Review:
  • If anyone knows sphinx/docutils well, take a look. But realistically, that may be no one so merge if CI passes.