fix: move and add back ariane-flight-v88 link

`introduction.md` was missing the link completely. In `about.md`, it was
defined outside of the admonition it was unsed in and therefore unavailable
from within the admonition. This is fixed by moving the link definition inside.

Author: ahans

concepts/pointers/about.md

@@ -63,6 +63,8 @@ openStarGate(dialedAddress);
 Pointer arithmetic in C++ can easily lead to __undefined behavior__ if not handled carefully.
 Undefined behavior can manifest in unexpected program outcomes, crashes, or even security vulnerabilities.
 One infamous example of the consequences of undefined behavior occurred in the [explosion of the Ariane 5 rocket][ariane-flight-v88] in 1996, where a software exception caused by the conversion of a 64-bit floating-point number to a 16-bit signed integer led to a catastrophic failure.
+
+[ariane-flight-v88]: https://en.wikipedia.org/wiki/Ariane_flight_V88
 ~~~~
 
 ## Accessing member variables
@@ -131,5 +133,4 @@ It is your responsibility to detect these cases and ensure those pointers are su
 In older code, you might encounter two alternatives to `nullptr`.
 Firstly, the literal `0` is specifically interpreted as a null value for pointers, though it's the only scenario where an integral literal can be assigned to a pointer.
 Secondly, the `preprocessor macro` `NULL`, inherited from C and defined in the `<cstddef>` header, is another representation of a null pointer, though its usage is less common in modern C++ code.
-[ariane-flight-v88]: https://en.wikipedia.org/wiki/Ariane_flight_V88
 ~~~~

concepts/pointers/introduction.md

@@ -63,6 +63,8 @@ openStarGate(dialedAddress);
 Pointer arithmetic in C++ can easily lead to __undefined behavior__ if not handled carefully.
 Undefined behavior can manifest in unexpected program outcomes, crashes, or even security vulnerabilities.
 One infamous example of the consequences of undefined behavior occurred in the [explosion of the Ariane 5 rocket][ariane-flight-v88] in 1996, where a software exception caused by the conversion of a 64-bit floating-point number to a 16-bit signed integer led to a catastrophic failure.
+
+[ariane-flight-v88]: https://en.wikipedia.org/wiki/Ariane_flight_V88
 ~~~~
 
 ## Accessing member variables