DOC: Add examples to Execution Configuration section in user manual

[remkop]

The user manual's section on Execution Configuration currently only shows a bullet list of the configurable items.

• ParameterExceptionHandler can point to existing section https://picocli.info/#_invalid_user_input
• ExecutionExceptionHandler can point to existing section https://picocli.info/#_business_logic_exceptions
• ExecutionStrategy can point to existing section https://picocli.info/#_initialization_before_execution
• ColorScheme can point to existing section https://picocli.info/#_color_scheme

Note that the Color Scheme section can be enhanced by showing the applySystemProperties() method that can be used to allow end users to override (parts of) the programmatically configured color scheme.