Make command line help easier to understand

[SaschaWillems]

Description

A common complaint, both here at github as well as social channels like Discord, is how to actually run our samples. You need to specify the name of a sample after an argument called sample. If you don't do that you get a huge list of all available options starting with an error text that's not easy to understand:

Not only is this a lot of text that'll confuse people, but the actual command on how to run a sample is somewhere at the bottom of this.

This PR simplifies all of this, and if you now run the application without a sample or with a sample name that doesn't exist you get a proper help text that's much easier to understand:

This message also tells people about the two other important arguments: getting a list of available samples and a list of all available command line arguments.

This should make it much easier to understand how to launch samples for new users.

Fixes #966

Tested on Windows 11 and Ubuntu 22.04 (via WSL2)

General Checklist:

Please ensure the following points are checked:

• My code follows the coding style
• I have reviewed file licenses
• I have commented any added functions (in line with Doxygen)
• I have commented any code that could be hard to understand
• My changes do not add any new compiler warnings
• My changes do not add any new validation layer errors or warnings
• I have used existing framework/helper functions where possible
• My changes do not add any regressions
• I have tested every sample to ensure everything runs correctly
• This PR describes the scope and expected impact of the changes I am making

Note: The Samples CI runs a number of checks including:

• I have updated the header Copyright to reflect the current year (CI build will fail if Copyright is out of date)
• My changes build on Windows, Linux, macOS and Android. Otherwise I have documented any exceptions

[tomadamatkinson]

LGTM!

[gary-sweet]

A very useful PR.