CONTRIBUTING.md: Rewrite and restructure introduction and getting started process

### Describe the issue

This Issue is part of an ongoing effort to simplify and improve the "Contributing to the AWS CDK" process. To start, we should improve the "Introduction" and "Getting started" sections by simplifying and revising content. For content that is important to know, but not necessary for everyone, we should move this content to a reference section. Also, my suggestion is that we remove the initial flowchart since the simplified process negates the need for it. Lastly, we should provide instructions for searching Issues and PRs and submitting Issues.

### Links

https://github.com/aws/aws-cdk/blob/main/CONTRIBUTING.md

Start by reading CONTRIBUTING.md, focusing on the Introduction and Getting started sections named in the issue. Review the existing flowchart and identify content that belongs in a reference section, then add guidance for searching Issues and PRs and submitting Issues. Done means the contribution process is simpler, reorganized, and covers the requested guidance.

Written by the indexing model from the issue text.