Improve documentation of new features in Changelog

## Desired behavior

Our Changelog currently has a list of pull requests included in each release with the pull request title, but it would be helpful to include more information about significant features included in each release. This could be included in the Changelog file itself and/or in the [GitHub release page](https://github.com/gazebosim/sdformat/releases).

## Alternatives considered

## Implementation suggestion

## Additional context