chore: import upstream snapshot with attribution

docs/experimental-editor-docs/getting-started.md

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+# Custom Template Import Getting Started
+
+This document explains the current PPTX import behavior from the custom
+template page.
+
+## What This Flow Does
+
+The `Select a PPTX file` action on:
+
+```txt
+/custom-template
+```
+
+uses the original Template Studio UX. The slide-editor import backend is always
+used.
+
+The flow is:
+
+1. Select a PPTX file.
+2. Click `Check Fonts`.
+3. Review available and missing fonts in the inline font management step.
+4. Upload any missing fonts you want to preserve.
+5. Continue to generate the old `Slide Preview` screen.
+6. Click `Generate Template` to create the reusable template.
+
+Template Studio still uses the same old UX: inline font management, slide
+preview, progress, and generated preview cards. Only the Generate backend
+changes. It sends the preview data to the Templates V2 API:
+
+```txt
+POST /api/v2/templates
+```
+
+That v2 request creates and saves the template, while the page stays on the
+old `/custom-template` preview experience. The generated preview cards render
+the raw Templates V2 `layouts.layouts[*]` JSON directly; they do not stage or
+open a slide-editor deck.
+
+Template Studio does not open the `Prepare fonts` dialog, create a slide-editor
+deck, stage anything in IndexedDB, or redirect to `/slide-editor`.
+
+## Verify The Flow
+
+1. Start the app normally.
+2. Open `/custom-template`.
+3. Click `Select a PPTX file`.
+4. Choose a `.pptx` file under 100 MB.
+5. Confirm the selected file appears in the upload card.
+6. Click `Check Fonts`.
+7. Confirm the inline font management step appears.
+8. Continue to preview.
+9. Click `Generate Template`.
+10. Confirm the page stays on `/custom-template`.
+11. Confirm Generate creates the template with `POST /api/v2/templates`.