[PR] Address BIOS/MBR issues in Prerequesites

[the-P1neapple]

Type

• Rewrite already written documentation.
• Add/remove new pages.

Questions

• Did you preview on your local machine beforehand?
• Did you follow our commit handbook?
• Did you read the Atlas Contribution Guidelines?
• Did you commit to the dev branch and not master?

Describe your pull request

I have seen several instances where people have INACCESSIBLE BOOT DEVICE BSODs after installing Atlas on BIOS-only systems. Creating this warning for people with older systems.

[the-P1neapple]

Feel free to make this it's own section if you feel it is needed.

[he3als]

MBR should work fine, if there's an issue with it in Atlas, that should be treated as an Atlas issue separately.

For Windows 11, that's covered in the system specifications pages, users will have to find a way to do it themselves.

Thanks for the contribution though.