Enhance CLI feedback

Description of the issue

The OMI CLI currently provides only limited feedback after running commands.

During testing, it was often unclear which files were created and what the next step should be. Users therefore need to manually inspect the generated folders.

More CLI feedback could make the workflow easier to follow, especially for new users.

Examples

After omi dataset:

• show a success message, e.g. Dataset successfully created
• show which dataset files were created
• optionally suggest omi resources as the next step

After omi resources:

• show a success message
• show how many CSV files were processed
• show which resource YAML files were created

After omi assemble:

• show a success message, e.g. OEMetadata JSON successfully created
• show where the JSON file was created
• optionally warn about missing metadata such as title, description or license