feat: add pointers concept

[vaeng]

I would put that concept after "references" and follow up with smart pointers. the trinity of confusion.

I think windowing system might be a fitting concept exercise.

[vaeng]

Pointers. Haiyaa.
It is difficult to make it short and interesting but not shallow.
Any suggestions?

[siebenschlaefer]

Not bad!

This whole documents describes pointers as "memory addresses". That is not wrong, at least on a practical level.

Formally (I think) neither the C++17 standard nor the C11 standard (which is part of the C++17 standard) define pointers in terms of "memory" but rather with "addresses" and "objects". Instead of defining what a pointer is they describe how you can derive it, when it is valid, how you can use it.
You could consider replacing many occurrences of "memory" in this PR with "objects", e.g. instead of "Pointers are a way to share memory" you could write "Pointers are a way to share objects". That would match the official terminology.

If you want to describe pointers as addresses in memory that's fine with me, just keep in mind that this is not in the official specification, and you cannot refer to some formal definition but have to describe it in your own words.

I'm missing a warning in the section about dynamic memory allocation that modern C++ code should use smart pointers (perhaps with a link to the other concept). I'd rather remove that whole section and put it into a different concept about dynamic memory allocation.

[vaeng]

@siebenschlaefer, if you think this is good to merge now, I will add the about.md file and make configlet happy.

[siebenschlaefer]

Looks good.

My new suggestions are mostly nitpicks, feel free to ignore or disregard them.

[vaeng]

I moved the section about dynamic memory allocation to the about.md and marked it as advanced.