Document the percent sigil

[samoylovfp]

Fixes #2166

Is the example too convoluted? Maybe just

let cmd = "is-empty"
[ 1 ] | %$cmd

?

[Juhan280]

Primary use case for % is to call builtins that are overriden by other custom command.

alias cd = __zoxide_z
%cd some_dir # can be used to run builtin cd

This can also be used for wrapping a builtin command with the same name

def ls [] {
  %ls | sort-by size
}

[fdncred]

This isn't a great example of why % exists. Maybe one like Juhan suggests would be better here.