add CONTRIBUTING.md && code of conduct

[alejandro-vaz]

add those two files, based on the @servo documentation

it's copy-paste https://servo.org/coc/ and https://book.servo.org/contributing/getting-started

both v1 and v2

[alejandro-vaz]

we should also add a notice about AI usage in contributions

I'm still strongly in favor of #447 zero AI, but we should also encourage people that their mistakes are okay and that we are all human beings and there's no shame in doing something wrong and learning from it

the amount of people using AI for many aspects of their life it should never be used in, and leaking private-ish information is insane

it just hit me recently how scary that is

and I'd love to say I'm not in that group, but I use AI for stuff in life you should never be using it too (not in PRs though)

in a weird way that many times doesn't come down to "you're lazy, that's why you're doing it with AI", it's more "I need confirmation about the way I'm doing something being right" and that's such a human need

maybe add something like that on the wiki too