Rollup merge of #156879 - apiraino:link-coc-rustfmt, r=ytmimi

Replace rustfmt code of conduct with link

In rust-lang/rust#65141 we replaced the CoC file and linked the "authoritative source" on the website. I noticed that the rustfmt directory still has the CoC original file.

I *think* for consistency it makes sense to have all of them linked but I'd like to hear from @calebcartwright first :-)

r? @Mark-Simulacrum

Author: JonathanBrouwer