Gentoo AMD64 Handbook "Preparing the disks" section reworked

Since the text was becoming more and more outdated and also more and more convoluted, I have completely reworked the "Preparing the disks" section of the Gentoo AMD64 handbook. 

• Since fdisk supports GUID partition tables (GPT) for a long time now, references to parted have been dropped.
• The text restricts itself now to the combinations 1) UEFI boot and GPT and 2) BIOS / legacy boot and MBR. While mixing and matching is here certainly possible, we should treat it out of the scope of the manual.
  
• Hopefully the terminology regarding the boot partition, UEFI system partition, and BIOS boot partition is more clear now (it was horribly mixed up before).

Please proofread and check for mistakes! I'll drop the "work in progress" label in a few days if nothing comes up.