docs: new quickstart guide

Created by: ryan-blunden

The one and only goal of the new quickstart guide is provide developers with a frictionless experience getting Sourcegraph configured with repositories cloned (enabled) and basic code intel enabled.

I want to get feedback from developers on this experience first before merging changes into index.md. I think the new admonition in index.md highlights the new quickstart guide's existence while making it clear its a WIP ("working on a new quickstart guide").

You can preview the changes at https://docs.sourcegraph.com/@new-3.0-quickstart