Correct Netlify section for the deploy-preview…

Details

- the synced free URL while a PR is open is deploy-preview-<PR#>--<site>,
  not v3--<site> (Netlify builds a PR'd branch in the deploy-preview context)
- document the actual working config (production branch = main, branch
  deploys = All, deploy previews = any PR) and that deploy previews + branch
  deploys are unmetered while only production deploys cost credits
- update the summary table accordingly
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01HShRpQUmDUttxD5F89ziDu

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