Contributor Onboarding Guide

### What is wrong?

There are two categories of currently implicit knowledge that would be useful for new contributors to know as early as possible.

First is the high level design of the Fe compiler itself, which includes subtopics like its architecture, extension points, and design philosophy (which can help new contributors make & evaluate changes accordingly).

The second is workflow considerations, such as code conventions, testing conventions (e.g., dealing with snapshots, adding error tests & acceptance tests, etc) and possibly useful guides that provide heuristics about where to start & the path to take when starting to add a new feature.

It can also be unclear *which* issues are more or less complex and, therefore, which are better suited for first time contributors.

### How can it be fixed

Create a contributor's guide (CONTRIBUTING.md) as well as auxiliary pages added to the documentation. Further, label appropriate issues with a "good-first-issue" label.

Start with the requested CONTRIBUTING.md and the existing documentation to identify where the Fe compiler architecture, extension points, design philosophy, coding conventions, testing workflows, snapshots, error tests, and acceptance tests belong. Done means a contributor guide and auxiliary documentation cover these topics, explain where to start on new features, and appropriate issues are labeled good-first-issue.

Written by the indexing model from the issue text.