Documentation: Convert the links in the .desktop file to absolute paths to avoid potential problems

[David-Else]

When following the documentation for installing the desktop files as it is, I encountered numerous issues where the Helix icon and Helix desktop integration would sometimes work and sometimes not. Diagnosing the root cause was a nightmare. Eventually, I discovered that the .desktop file needed absolute paths to the Helix binary and icon.

Note that kitty asks users to do the same thing: https://sw.kovidgoyal.net/kitty/binary/#desktop-integration-on-linux

# Update the paths to the kitty and its icon in the kitty desktop file(s)
sed -i "s|Icon=kitty|Icon=$(readlink -f ~)/.local/kitty.app/share/icons/hicolor/256x256/apps/kitty.png|g" ~/.local/share/applications/kitty*.desktop
sed -i "s|Exec=kitty|Exec=$(readlink -f ~)/.local/kitty.app/bin/kitty|g" ~/.local/share/applications/kitty*.desktop

I am aware that the next section To use another terminal... assumes that the user has not made this absolute path change, but it might confuse things further to add it there. I am assuming that users could work out what is going on, it would be overkill to add all possible combinations, but I would be interested in some feedback on that.

[the-mikedavis]

I think this is ok as-is, I don't think it's too confusing that these two sections conflict a little. We can update this section later if it's confusing and someone has a suggestion