Split the content styles generator script

[pomek]

Provide a description of the task

Currently, the Content styles generator detects new plugins (introduced between the script executions) and prepares the content styles (https://github.com/ckeditor/ckeditor5/blob/master/docs/builds/guides/integration/content-styles.md).

To use the {@exec...} tag, the script must be split into two parts:

• a script that detects new plugins - it will be called once per a release
• a script that prepares the content styles CSS file

Once it's done, we can use {@exec...} for reading the CSS file and inserting its content directly in the guide.

[pomek]

TODO: Review release notes (notion).

[pomek]

• The script for displaying new plugins should ask about committing the updated plugin database.

[pomek]

TODO: Review release notes (notion).

Done.