sample of string interpolation and evaluation inside a string for echo command

[jordanst3wart]

I thought I'll add an example of string interpolation on the echo command page, as it's a common use case coming from other shells, ie. bash/zsh.

[ysthakur]

The text "string interpolation" could maybe link to the docs on string interpolation in case people want more in-depth information on how it works

[jordanst3wart]

Yeah, that's a great idea ysthakur

I think i need to add an examplenushell/crates/nu-cmd-lang/src/core_commands/echo.rs

[jordanst3wart]

implied at: https://github.com/nushell/nushell/pull/12051

[fdncred]

Command help text isn't allowed to be changed since it's systematically generated with each release.