Add code for insertion sort in cpp

[TanushreeSB]

Please assign me this issue @extinctsion under hacktoberfest 2025.

[extinctsion]

Hi @TanushreeSB this repo is currently excluded from hacktoberfest. You can find link to another repo from the readme. I request, this hacktoberfest you create value PRs rather than just DSA questions. If you are not new, I highly recommend to explore my other repos which are labelled hacktoberfest. Let's connect on LinkedIn to know more.