docs(example): Add minimal slash command (ping) example.

[kaedeek]

Summary

Add a minimal slash command (ping) example.

Additional context

While similar functionality is shown in the README, this example provides a minimal, runnable version for beginners.

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[ruslan-ilesik]

Hey, thanks for PR, while I am not the main reviewer I would like to give a suggestion.

Would be nice if you could also put usage of

https://dpp.dev/classdpp_1_1cluster.html#acb7b73536f793da2a2955d70baa991f1

Into the guide, as better alternative to if else in on_slashcommand.

[Ashthetik]

I'm not much of a contributor, but what would be the purpose from moving the ping example from https://dpp.dev/firstbot.html to the example programs?

[ruslan-ilesik]

Also, would be great if you had comments in code itself quickly explaining what each of lines do, for new people to c++ it may be very helpful

[ruslan-ilesik]

I'm not a contributor, but what would be the purpose from moving the ping example from https://dpp.dev/firstbot.html to the example programs?

Also a good point, but we have it as first bot AND kinda advertisement like "look, it is not harder than js", so I am not sure about that. @braindigitalis should be judge of it.

But in any case, either keep it in examples or update main page to more "fresh" implementation may be beneficial