Allow clearing content without clearing configs

[aerni]

I think it would be useful to enhance the site:clear command with the option to only clear actual content without clearing configs. For example, it should only clear the entries in content/collections/pages and leave the content/collections/pages.yaml config untouched.

This feature would be particularly useful for content-heavy starter kits. Developers could provide a default set of content for users to get familiar with the starter kit. Once the user is ready to roll their own content, they can easily clear the demo content without purging the whole setup of collections, config files, and views.