[zimeg]

Summary

This pull request adds the checkboxes element example as a modal view with a section block holding a checkboxes accessory, matching the docs payload.

• Adds Checkboxes.example01() with a test asserting the docs JSON, plus a README entry in docs order.
• The second option uses a mrkdwn description, which the option object docs allow for checkbox groups.
• CI fails until feat: support mrkdwn descriptions on OptionObject (#1471) slackapi/java-slack-sdk#1645 is released and the slack-api-model pin is bumped: OptionObject.description is PlainTextObject in 1.54.0, so markdownText(...) doesn't compile yet. The example is written as the docs need it, without a cast or workaround.
• The docs JSON has a trailing comma after the A2 description, which makes it invalid. The test's expected JSON leaves it out.

Testing

1. Once #1645 is released, bump slack-api-model and slack-api-client in block-kit/pom.xml.
2. Run mvn compile and mvn test in block-kit.
3. Paste the expected JSON from CheckboxesTest into Block Kit Builder in modal mode and confirm option two shows a bold description.

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