add disable secure boot instructions

[sssoleileraaa]

Description

Add instructions to disable Secure Boot (until we make this so that it's no longer a requirement).

Can't remember if I had to do this for nuc7, but it might make sense to move the instructions to the top as a check for all nucs.

• Fixes Add screenshots for disabling secure boot on nuc8 #151

[zenmonkeykstop]

I think it's good as is - the pending PR for focal installs includes a general section on BIOS settings (with Secure Boot info), and the NUC7s will probably age out of the reccos list soon.

[eloquence]

git checkout on this branch with LFS installed notes that the binary is not tracked via LFS:

Encountered 1 file(s) that should have been pointers, but weren't:
	docs/images/hardware/nuc8_visualbios_secureboot.png

If LFS is installed, I believe git add and amending the existing commit should resolve.

[eloquence]

LGTM, thanks for resolving LFS issue :)