docs: Added description of heat map interpolation into HeatmapRendering.md

[HaimingXu679]

I added a short description to HeatmapRendering.md with an image of a sample heat map result. The markers (4) indicate the four data points given. Every other point in the heat map was artificially created.

[arriolac]

Thanks! Just one comment

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[googlebot]

CLAs look good, thanks!

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[googlemaps-bot]

🎉 This PR is included in version 3.3.0 🎉

The release is available on GitHub release

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