docs: update CONTRIBUTING with new flow

[jsjoeio]

This PR updates CONTRIBUTING.md with our new development workflow.

Fixes #4478

[github-actions]

✨ Coder.com for PR #4502 deployed! It will be updated on every commit.

• Host: https://codercom-ijhyoyy1m-codercom.vercel.app/docs/code-server
• Last deploy status: success
• Commit: 790ccfb
• Workflow status: https://github.com/cdr/code-server/actions/runs/1463349678

[codecov]

Codecov Report

Merging #4502 (790ccfb) into main (e705948) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##             main    #4502   +/-   ##
=======================================
  Coverage   65.93%   65.93%           
=======================================
  Files          30       30           
  Lines        1638     1638           
  Branches      330      330           
=======================================
  Hits         1080     1080           
  Misses        475      475           
  Partials       83       83           

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