suggest *.desktop for AppImage

[Rudxain]

Since the official AppImage doesn't include the icon/logo/isotype, and some desktop environments (such as GNOME) don't allow executing arbitrary files by double-clicking (for security reasons), I believe it's beneficial for the user to have the .desktop shortcut file

[Rudxain]

BTW, it would be nice if shell completions were documented, because AppImage users might want it, and (AFAIK) following the docs doesn't install any completion

[Rudxain]

related: #4159

[krish-r]

Minor suggestions: (Please feel free to ignore)

• It'll be better to avoid the temp variable ($d) to avoid any potential overwrites.
  (Also, fish for ex. does not support var=val or unset)

• Instead of suggesting this as a pre-requisite for desktop shortcut, can this be something like -
  "To run / start Helix from anywhere, make sure to add it to your $PATH".

• Also, I think it'll be better to keep the same link text for consistency (might also be easy during the search) -
  "You can also optionally Configure the desktop shortcut".