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
}

[samoylovfp]

Now that I understood the intention, I think I found a better place for this piece of information.

[fdncred]

I think it's fine here but the it shouldn't be the only place because I didn't write the code for aliases. I wrote it to be able to run built-ins from anywhere. So, either that's another special section in another area or it's around where ^ is documented.

[samoylovfp]

@fdncred
I've added mentions of the percent sigil to both the section about overriding commands and one about executing strings.
Does this look better?

[fdncred]

I think it's fine but I'd do something like this just to drive the point home.

# call internal `ls` command
%$cmd
# call external `ls` command
^$cmd

[Juhan280]

Nvm, I didn't expand the diff, and just assumed the wrong thing