Update contribution docs

[Jay-Plumb]

PR Summary:

1. Updates both README.md and CONTRIBUTING.md with more up to date links relating to the shopify cli for local development.
2. Resolve console deprecation warning:

The `shopify theme serve` command is deprecated.                  
                                                                                       
Use `shopify theme dev` instead.    

Why are these changes introduced?

Fixes #2974

What approach did you take?

Went through the process myself as it's the first time spinning up the repo locally

Visual impact on existing themes

No visual impact as it is setup related

Checklist

• [] Added PR summary for release notes
• Created a ticket for the help.shopify.com documentation team about updates to theme settings. (Internal-only task)
• Followed theme code principles