✨ FEAT: Update Templates with the Thumbnail template stored in assets

[njtalba5127]

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template thumbnail in templates are not yet linked to the thumbnail inside the assets folder in our repo.

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instead of using the image stored in github user content, for easier updating in the long run, better to instead get the image from inside the assets folder of the repo.

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well if thats no gud, but im sure it would work, then stay with what we have :/

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<a href="https://github.com/nejilabs"><img src="https://raw.githubusercontent.com/nejilabs/nnist-pt-20230125/main/.github/assets/thumbnail-template.png" width="100%" /></a>

then for generating projects from this template, changing the thumbnail.png in the assets folder and linking to that image instead would be much easier.