docs: add a CLI setup recording to the quickstart and a dashboard quickstart page

[JoshKappler]

Summary

• Embeds a 57s terminal recording of the full CLI setup (install, npx gt init, credentials, npx gt translate, real output) at the top of the CLI quickstart.
• Adds platform/quickstart.mdx, a linear Dashboard setup path with a 37s walkthrough video, plus a Quickstart card on the platform index and the get-started walkthrough embed on platform/dashboard/get-started.mdx (theme-matched, its own light and dark takes).
• Pure content change. Recordings live in the media repo and serve from assets.gtx.dev/walkthroughs/; the player renders in gt-cloud's landing app (gt-cloud#3617). Here Asciinema registers as a StubComponent, the existing convention for landing-only components.
• Net diff vs main: the MDX pages, the index card, meta.json, one stub registration line, and optional muted/srcDark props on the existing Video. No new components, dependencies, or config. Video renders only the light source here, since this app has theming disabled and a dark variant could never show.

Testing

• pnpm build:content green (every new route prerenders through the stub); validate:unsafe-html, validate:callouts, validate-links, and the validator self-tests all clean.
• The real player render is verified on gt-cloud#3617; this renderer is preview-only.
• Two-reviewer adversary pass across the stack.

Notes

• Rework per @pie575's review (7/21): media moved to the media repo, Lottie dropped, all player code removed here, so content stays pure MDX. Two video feedback rounds followed on 7/22: re-recorded with the general-translation example project from an empty org at 1.15x, page-anchored asset names, assets reorganized under assets/walkthroughs/, default black cursor and no blur (the revealed key is a mock swapped in before paint).
• Merge order: media#5 first (it uploads the files), then gt-cloud#3617, then this, then the content submodule bump in gt-cloud. Previews 404 until media#5 lands.
• The related frontmatter on the new page renders in landing only; apps/content strips it for every docs page today (pre-existing schema gap, not introduced here).

cli-setup.mp4

dashboard-setup.mp4

[pie575]

Same complaints as I had in https://github.com/generaltranslation/gt-cloud/pull/3617

More questions: Why are we duplicating the mp4 file to two repos? Same with the Lottie, Asciinema, and .cast files. This seems like bad logic.

Again, we should be moving things to use the new Media repo without strange cross repo copied logic, that is bound to end up out of sync.
If things are unclear, we should instead look into how we did videos in the past not like this