always serve the index.html on netlify

[mansona]

It turns out that going directly to https://blog.emberjs.com/page/1 doesn't work 🤔 this is because we don't have a fallback HTML for netlify

This PR fixes that.

To test:

• load the netlify preview for this PR
• scroll to the bottom and click "older articles"
• refresh

if it shows you the 404 page from netlify then this didn't work

[netlify[bot]]

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[locks]

@mansona mergy mergy?