Added some docs and an example for how to use the APIC

[Voinea-Radu]

I do believe that this could be useful when migrating from version 0.9 to version 0.11, as well as a quick way to get things going if starting from nothing.

[Freax13]

Thank you for your contribution!

I'm not sure if we should adopt this example: This example goes way beyond what's needed to get started in setting up a small kernel with the bootloader. I'm not sure if we want this repo to serve as a place for various examples of next-steps one could take in kernel dev.

Perhaps this is better suited for the x2apic crate. Maybe it could live in its own repo entirely.

[Voinea-Radu]

Hi there, thank you a lot for taking the time to look into this
I do understand that it might be a bit complex for a small kernel, but these were what I needed to port the 0.9 blog version to the 0.11 current version and would have liked to have had them just be available somewhere so I can read thru them. I do find that especially some of the structs that hold all the values are unnecessary for a small startup and would be more than happy to remove them along side any other future proofing