Fix elvish sudo integration and update documentation

[Yappaholic]

Elvish integration is broken when running sudo, because the function sudo-with-terminfo uses command command, which is not implemented in Elvish. Changing it to external command should fix possible error when bypassing aliases, functions and builtins, like command does in Bash.
Discussion about this issue: #5979

Also I updated documentation about Elvish integration to provide fix when using use ghostty-integration outside of Ghostty with rc.elv.

[mitchellh]

I don't use elvish but googling backs this up and the docs improvements are appreciated.