docs(core): fix relative images for previews on vercel

[jaysoo]

This MR fixes relative image paths nx.dev docs.

For example on /preview/react/guides/nextjs.

[vercel]

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🔍 Inspect: https://vercel.com/nrwl/nx-dev/ArVivJ4CrVEomcbZB878Gu1WqM9H
✅ Preview: https://nx-dev-git-fork-jaysoo-nxdev-fix-vercel-preview-images-nrwl.vercel.app

[nx-cloud]

Nx Cloud Report

CI ran the following commands for commit 5066a07. Click to see the status, the terminal output, and the build insights.

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Status	Command
	nx affected --target=e2e --base=cddb1edd5670e6ac7b106acc427dc23117cc2740
	nx affected --target=lint --base=cddb1edd5670e6ac7b106acc427dc23117cc2740 --parallel --max-parallel=4
	nx affected --target=test --base=cddb1edd5670e6ac7b106acc427dc23117cc2740 --parallel --max-parallel=2
	nx affected --target=build --base=cddb1edd5670e6ac7b106acc427dc23117cc2740 --parallel --max-parallel=3
	nx build typedoc-theme

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[github-actions]

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