loader.efi: Improve the manual page

• Add SPDX tag
• Add a files section describing different locations related to the loader
• Add an example explaining how to install a new loader.efi to ESP
• Reference uefi(8)

Reviewed by: tsoome
Reviewed by: Pau Amma <pauamma@gundo.com>
Reviewed by: Jose Luis Duran <jlduran@gmail.com>
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D34592