x/website: add Gerrit basic tutorial in contributing guidelines

[dolmen]

This is about https://go.dev/doc/contribute#review

As long time Open Source contributor and heavy GitHub user, I struggle with the contribution process for Go because of the Gerrit workflow and UI. The Gerrit UI and process is very different from GitHub. For example, I got patches stuck for weeks just because the answers to reviews I had written were just drafts and not sent to the reviewer.

So it would very helpful to add:

• some basic tutorial for the contributor point of view, with screenshots:
  • how to reply to comment
  • the workflow state schema
  • what to do when a patch in "Your turn" state
• links to a Gerrit UI onboarding

This presentation seems to have good in-depth content, but goes too deep for someone who just want to send one patch to the Go project.
https://docs.google.com/presentation/d/1C73UgQdzZDw0gzpaEqIC6SPujZJhqamyqO1XOHjH-uk/edit#slide=id.g4d6c16487b_31_60

[dmitshur]

It's worth taking into account that Gerrit itself has fairly thorough documentation (e.g., see https://gerrit-review.googlesource.com/Documentation/index.html).

For example, https://gerrit-review.googlesource.com/Documentation/user-attention-set.html explains the "Attention Set" feature in depth, and https://gerrit-review.googlesource.com/Documentation/user-review-ui.html goes over all the UI elements that can be useful during code review.

The https://go.dev/doc/contribute#sending_a_change_gerrit section currently explains step by step what it takes to send a change via Gerrit, but it can't cover everything Gerrit offers without also becoming as large as the links I shared above. Gerrit itself is improving and sometimes gets new features (e.g., Attention Set was only added in recent years).

The point I want to make with this is that it could work well to take advantage of Gerrit's own existing documentation in some helpful way, and avoid trying to replace it.

[mknyszek]

As an easy first step, we could probably just add those links to the page.