expanding snap store page

[whoa220]

I have expanded snap.rst to explain how to install software on linux mint. I have included

• An explanation of the software manager and how it works
• How to change software sources
• how to use APT terminal program
• explaining how .deb files work

[whoa220]

these three pulls are my first ever contributions (and actually using git semi-seriously) to any major project like this.
could people please tell me if I did anything wrong with this?

[Jeremy7701]

It's really useful that someone has finally started producing some top quality user documentation.
The only suggestion I'd have is to possibly change Line 12 of docs/nvidia.rst
from
LANG=C inxi -Fxxxrzc0 --usb
to
LANG=C inxi -Gxxxc0
(In the interests of reducing the amount of output).