Remove contributing section from docs

[Dedelweiss]

All pages in the "Contribute" section have been added or not (if the information was already present). The purpose of this PR is to gather in one place the information in order not to lose the user wishing to get information in the documentation.

Fixes: #15111

[dwijnand]

I disagree with #15111 - we should keep contributing in the repo, where the contribution happens. So that when technical details change the docs can be updated at the same time. So I'd say we should remove it from https://docs.scala-lang.org/scala3/guides/contribution/contribution-intro.html, making that point to the repo's docs.

[bishabosha]

So I'd say we should remove it from https://docs.scala-lang.org/scala3/guides/contribution/contribution-intro.html, making that point to the repo's docs.

ok but the sources on the docs website are more up to date, (and rewritten to be user friendly) so it should be copied over if that's the decision to take