Add ignore clause to netlify builds

[ppbaena]

Description of the change

Netlify builds often fail because there is no new content added to the website. It compares the cached version with the PR's (to main) content and given there are no changes, no deploy action is performed. Adding an ignore clause to netlify.toml file to prevent this situation, so builds only proceed when changes in the base directory (/site).

Benefits

• Save some building cycles for netlify.
• Avoid the "failed" badge for netlify on top of our readme.

Possible drawbacks

N/A

Applicable issues

• fixes Netlify reporting "failed" status main's pushes when hitting a cached version #4878

Additional information

[netlify]

✅ Deploy Preview for kubeapps-dev ready!

Built without sensitive environment variables

Name	Link
🔨 Latest commit	ee104a0
🔍 Latest deploy log	https://app.netlify.com/sites/kubeapps-dev/deploys/62cd5e58b5af8b0009383b98
😎 Deploy Preview	https://deploy-preview-5065--kubeapps-dev.netlify.app
📱 Preview on mobile	Toggle QR Code... Use your smartphone camera to open QR code link.

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[ppbaena]

See netlify docs and example 5 in this support question mentioning the same use case we're looking for (meaning deploy only changes inside /site folder).