Add both dark and light theme logos

[BRO3886]

Reference: https://docs.github.com/en/github/writing-on-github/getting-started-with-writing-and-formatting-on-github/basic-writing-and-formatting-syntax#specifying-the-theme-an-image-is-shown-to

[saikun0293]

The current gdsc logo looks good in both light and dark. There isn't any need for alternating logos anymore and hence closing the issue.