[DMN Designer] Documentation - Implement styles and presentation enhancements

Currently, we're not showing any image in the documentation cover (in the DMN Documentation).

Follow UX team input (https://issues.jboss.org/browse/DROOLS-4245) to implement the enhancements.

This JIRA comprehends:

• The addition of the cover image (without any logic, since the same image will be used by productized and community version);
• The implementation of the CSS for all sections of the documentation.

Acceptance test

• Check logo on Chrome
• Top Level Data type highlight on Chrome
• Check logo on Firefox
• Top Level Data type highlight on Firefox
• Check logo in downloaded documentation
• Check top level data type highlight in downloaded pdf