feat: Code of Conduct page

[mkovaxx]

Applying for a Rust Foundation grant requires us to have a Code of Conduct.

For events hosted previously at the CyberAgent office, we have used their Code of Conduct, which is here:
https://cyberagent.events/code-of-conduct

I propose that we adopt it to be our official Code of Conduct for now, and have a translation in English as well.

Ideally, the page would be available at https://tokyorust.org/code-of-conduct.