Add Dark Mode & Instagram Handle on Socials

[unnati-joshi]

I would like to propose two feature additions to the Rust website:

1. Dark Mode Support
2. Add Instagram to Social Links

[fmease]

@rustbot transfer www.rust-lang.org

[rustbot]

Failed to transfer issue:
error: Could not resolve to a Repository with the name 'rust-lang/www'.

[fmease]

Please report these two issues over at https://github.com/rust-lang/www.rust-lang.org/issues instead but make sure they haven't been reported already

[workingjubilee]

I don't believe we have an instagram account.