Rename Ads architecture diagram to bust stale CDN cache

[MaartenRimaux]

Summary

The reworked Ads architecture diagram (#817) kept the same static filename how_ads_works-{light,dark}.svg, so the CDN layer in front of the site (which caches static assets with max-age=31536000 and isn't purged by the CloudFront invalidation in the deploy workflow) keeps serving the old signaling-service diagram.

• New v2 diagram renamed to how_ads_works-v2-{light,dark}.svg (a never-cached URL) and ads/index.mdx updated to reference it.
• The old diagram is restored at the original /ads/img/how_ads_works-{light,dark}.svg URL, moved from ads_versioned_docs/version-v1/static/ — that directory isn't in staticDirectories, so the v1 docs pages (which still reference the original URL) were incorrectly showing the v2 diagram; they now correctly show the v1 diagram again.

No other static assets were modified in place by #817, so this is the only asset with the stale-cache conflict.

Link to Devin session: https://dolby.devinenterprise.com/sessions/e5d8f394b68b4b049c0ee8f8ab076bdc
Requested by: @MaartenRimaux

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