docs: admin-guide: Update bootloader and installation instructions

Updates the bootloader and installation instructions in
admin-guide/README.rst to align with modern practices.

Details of Changes:

 - Added guidance on using EFISTUB for UEFI/EFI systems.
 - Noted that LILO is no longer in active development and provides
   alternatives.
 - Kept LILO instructions but marked as Legacy LILO Instructions.
   Suggest removal in future patch.

Signed-off-by: Hunter Chasens <hunter.chasens18@ncf.edu>
Signed-off-by: Jonathan Corbet <corbet@lwn.net>
[jc: repaired added whitespace warnings]
Link: https://lore.kernel.org/r/20240207171007.45405-1-hunter.chasens18@ncf.edu