Add symlinks for code of conduct and contribution guidelines (#5447)

Addresses
[this](https://github.com/paritytech/eng-automation/issues/10#issuecomment-2286391379).

> Another side note is the checkmarks in
https://github.com/paritytech/polkadot-sdk/community. It seems like even
though we have `CONTRIBUTING.md` in `/docs` it is not picked up in this
list. I am not even sure what is the benefit of following these
standards, but it is probably for the best to do it.