Plan publishes versioned static HTML.

Create a stable document:
  curl --fail-with-body --request POST --data-binary @plan.html \
    --header "Content-Type: text/html; charset=utf-8" \
    --header "Authorization: Bearer $PLAN_TOKEN" \
    https://plan.ellep.dev/documents

Update the returned URL with an authenticated PUT. Public GET and HEAD requests
always resolve it to the current immutable revision.