📝[Docs]: Add Day 8 Content (Loops in C++)

[AnshikaAg03]

What Docs changes are you proposing?
I would like to add for loop, while loop and dowhile loop in day 8

[github-actions]

Thank you for creating this issue! We'll look into it as soon as possible. Your contributions are highly appreciated! 😊

[AnshikaAg03]

@subhadipbhowmik please assign me this issue

[lovelymahor]

@subhadipbhowmik, Please assign it to me I want to work on this issue. Also, please add the label GSSOC and the level that you find appropriate for this issue. thank you!