feat: add smart pointer concept

[vaeng]

This is long, but I think it is pretty straightforward. Maybe I have tunnel vision and this is not readable at all :D

[vaeng]

About.md will follow when the main text has been reviewed.

[siebenschlaefer]

Not bad!

This description focuses on the deallocation. But IMHO an equally important aspect is that the objects gets properly destructed.

A large part of this PR describes weak pointers. Did you consider omitting them? For somebody who just learns about smart pointers std::unique_ptr is the most important thing, std::shared_ptr is a clear second, but std::weak_ptr is IMHO far less important. Dedicating a large portion of this document to std::weak_ptr does not reflect that.

This PR describes the motivation and technical aspects of the three smart pointers. You could consider adding some advice: When raw pointers are fine to use (if they are not owning), which smart pointer should be used when (by default: std::unique_ptr).

[siebenschlaefer]

I like it, I don't even have anything to criticize.
The only think I'm missing is the reference to works of fiction.

[vaeng]

The only think I'm missing is the reference to works of fiction.

Apart from king Arthur there is also "The Expanse" in there.