Add body from the milestone in the release block

When the option `data-source` is `milestone`, we can add the milestone description to the release body.

Start by tracing the `data-source` option handling for the `milestone` value and the release block that builds the release body. Verify how the milestone description is retrieved, then confirm that the description appears in the generated release body when this option is selected.

索引モデルが issue の本文から書いたものです。