chore: add back ignore check to allow deploys when changing plugin without changing demo site

[pieh]

Summary

PRs that don't touch demo site don't produce actual preview deploys for testing due to:

11:13:36 AM: No changes detected in base directory. Returning early from build.
11:13:36 AM: Failed during stage 'checking build content for changes': Canceled build due to no content change

The ignore setting was commented out in #547 and this PR checks wether it's safe to bring it back. Alternatively maybe https://answers.netlify.com/t/builds-cancelled-for-a-new-branch-due-to-no-content-change/17169/2 could be used to force the deploy everytime.

This PR also includes changes from #557 to be able to test them with actual Preview Deploy

Test plan

1. Visit the Deploy Preview (insert link to specific page) ...

Relevant links (GitHub issues, Notion docs, etc.) or a picture of cute animal

Standard checks:

• Check the Deploy Preview's Demo site for your PR's functionality
• Add docs when necessary

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🧪 Once merged, make sure to update the version if needed and that it was
published correctly.

[netlify]

✅ Deploy Preview for netlify-plugin-gatsby-demo ready!

Name	Link
🔨 Latest commit	51d2f4e
🔍 Latest deploy log	https://app.netlify.com/sites/netlify-plugin-gatsby-demo/deploys/63ef61d723c7170008523d9b
😎 Deploy Preview	https://deploy-preview-565--netlify-plugin-gatsby-demo.netlify.app/blog
📱 Preview on mobile	Toggle QR Code... Use your smartphone camera to open QR code link.

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