De-duplicate CONTRIBUTING.md and wiki page on contributing

I've recently made [a set of updates](https://github.com/adoptium/infrastructure/pull/3219) to the CONTRIBUTING.md ([GitHub standard](https://docs.github.com/en/communities/setting-up-your-project-for-healthy-contributions/setting-guidelines-for-repository-contributors#adding-a-contributing-file)) but we also have a "How to contribute to infrastructure" page in the wiki. We should de-duplicate these and probably standardise on the CONTRIBUTING.md which is where people typically expect to find this information.