Clarify Document Authoring vs AEM Sites content source

[sirugh]

Purpose of this pull request

Adds contextual callouts across four pages to distinguish the Document Authoring (DA.live) path from the AEM Sites path when using Edge Delivery Services. Readers who land on boilerplate or get-started content for a DA.live project were getting confused when their project uses AEM Sites as the content source, since the instrumentation, setup steps, and Universal Editor workflow differ.

Changes:

• boilerplate/universal-editor.mdx — note clarifying this page covers the DA.live content source; links to aem.live UE blocks guide for AEM Sites projects
• get-started/before-you-start.mdx — DA.live access row now notes it applies to the Document Authoring path only
• get-started/create-storefront.mdx — note pointing AEM Sites users to the XCOM boilerplate and EDS+AEM docs on Experience League
• merchants/quick-start/universal-editor.mdx — tip callout summarizing both authoring approaches and linking each to its respective docs

Affected pages

• https://experienceleague.adobe.com/developer/commerce/storefront/boilerplate/universal-editor/
• https://experienceleague.adobe.com/developer/commerce/storefront/get-started/before-you-start/
• https://experienceleague.adobe.com/developer/commerce/storefront/get-started/create-storefront/
• https://experienceleague.adobe.com/developer/commerce/storefront/merchants/quick-start/universal-editor/

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Pages that may need follow-up

A broader audit identified additional pages that are DA-centric without a scope callout. AEM Sites users landing on any of these could be misled, since block-table authoring, document metadata, and UE instrumentation all work differently when AEM Sites is the content source.

Priority	Page	Live URL
High	Block table structure	merchants/blocks/block-tables
High	Author and developer tasks	merchants/blocks/author-and-developer-tasks
High	Your first page (tutorial)	merchants/quick-start/your-first-page
High	What is Commerce Storefront?	merchants/quick-start/create-content
Medium	Page metadata	merchants/blocks/page-metadata
Medium	Section metadata	merchants/blocks/section-metadata
Medium	Personalization	merchants/content-customizations/personalization
Medium	Experiments	merchants/content-customizations/experiments
Medium	Multistore setup	setup/configuration/multistore-setup

The common fix for each is a short Aside at the top (same pattern as content-localization.mdx) scoping the page to Document Authoring and linking AEM Sites users to the equivalent AEM docs.