Show Back-to-Top Button Only After Scrolling some distance down

Currently, the back-to-top button appears immediately when the page loads, which can be improved further. To improve the user experience, I would like to update the button's behavior so that it only becomes visible after the user scrolls down a certain distance (e.g., 200px).

@Sanskriti1102 Kindly assign me this issue under the gssoc and hactoberfest labels