Port contributor guide from etcd-io/etcd to website

Copy (or move) the contributor guide from the @etcd-io/etcd repo to the website to surface information on how the project would like contributions to be made.

Source: https://github.com/etcd-io/etcd/blob/main/CONTRIBUTING.md

Potential strategy:
- [ ] Create a **Contributor guide** page -- either as a new page, or embedded in the [Community Page](https://etcd.io/community/).

Additional related work:
- [ ] Update and move **[Issue Triage Guidelines](https://etcd.io/docs/v3.5/triage/issues/)** under Contributor guide.
- [ ] Update and move **[Reporting bugs](https://etcd.io/docs/v3.5/reporting_bugs/)** page under Contributor guide.