bug#47630: Improve guix manual installation docs

[bo0od]

Hi There,

Checking here:

https://guix.gnu.org/manual/en/html_node/Manual-Installation.html

This section actually misses alot of commands and its not nice to give 
only hint commands and leave step by step explanation e.g:

https://guix.gnu.org/manual/en/html_node/Keyboard-Layout-and-Networking-and-Partitioning.html

"parted /dev/sda set 1 esp on"

This command wont work without labeling the partition "parted /dev/sda 
-- mklabel gpt" and so on..

This mean user cant copy and paste he need to figure out the errors 
while hes follow the documentation because it doesn't give to the user 
the full picture.

I suggest to take a look at NixOS docs which goes step by step without 
jumping any necessary one:

https://nixos.org/manual/nixos/stable/index.html#sec-installation-partitioning

(Unclear documentation causes unsolvable or ambiguous questions from 
users and its troublesome to the support team to re-add the missing 
parts of the docs in their support.(Assuming they figured out where is 
the issue user is falling in.))

ThX!