Changelog.md - Follow Markdown liting

Hi team 👋

I'd like to suggest an enhancement to the changelog generated by release-please-action. Currently, the output does not consistently follow Markdown linting best practices, which can lead to readability issues and problems with automated formatting tools.

Specifically, it would be great if the changelog adhered to the following markdownlint rules:

MD034/no-bare-urls: Avoid bare URLs; use Markdown links instead.
MD012/no-multiple-blanks: Prevent multiple consecutive blank lines.
MD024/no-duplicate-heading: Avoid duplicate headings at the same level.
MD032/blanks-around-lists: Ensure proper spacing around list items.

This would improve consistency and make the changelog easier to read and process in downstream tools.

Would you consider adding markdownlint compliance to the changelog generation logic or providing a configuration option for it?

Thanks for your work on this great tool!