Todo squash and better apt and systemd usage in instruction

[StephDC]

• L28: minit user would be automatically created by the postinst script of MinKNOW, thus not necessary.
• L34: the home directory does not seem to matter as long as we are at some home
• L51: apt-key is deprecated. Use /etc/apt/trusted.gpg.d instead
• L112: Use sudo -e to let the user edit the file, instead of a simple cat to show content
• L128: Instead of replacing the whole minknow unit file, use override to only override the user/group settings would be better and survive future minknow upgrades

I just assisted someone to follow this manual to install MinKNOW on Jetson, and it worked fine. Thanks for all your work, and there are a few things I confirmed and modified during the process, with my understanding to those Linux tools.