COMDOX-1472: Add ACCS onboarding documentation

[bdenham]

Purpose of this pull request

This pull request addresses documented gaps identified in the ACCS onboarding checklist gap analysis (COMDOX-1472). It adds six new pages and enhances two existing pages so readers can follow the full ACCS onboarding journey end to end. It also updates the Storefront Setup sidebar, cross-links from prerequisites, and includes the ProseFlow component so new pages have consistent typographic rhythm and the change set is self-contained against the release base branch.

New pages

• Pre-onboarding — Stakeholder alignment, business/use-case definition, architecture planning, infrastructure readiness, compliance review, and a technical access checklist.
• Experience Cloud access — Adobe Experience Cloud access, Admin Console setup, role-to-responsibility matrix, and how services connect to the EDS storefront.
• Commerce Cloud Manager — Credit validation, instance creation (production and staging), and environment ID retrieval.
• App Builder — Workspace setup for storefront event handling and extensibility.
• API Mesh — Configuration, when to use it, CDN/Fastly considerations, and VCL workarounds for header limits.
• Custom frontend — Custom React/Next.js guide: drop-in packages, import maps, SDK initialization (initializers.register()), and provider.render() mounting patterns.

Enhanced pages

• Content delivery network — DNS and custom domains (record types, CNAME/apex guidance, TLS management).
• Configuration hub — Callout directing ACCS users to the pre-onboarding checklist.

Other changes

• Sidebar entries for Pre-onboarding, Experience Cloud access, Commerce Cloud Manager, App Builder, API Mesh, and Custom frontend under Storefront Setup.
• Cross-links in prerequisites to pre-onboarding and Experience Cloud access.
• ProseFlow.astro copied from ia-updates for use on new pages.

Associated JIRA ticket

COMDOX-1472

Affected pages

• https://experienceleague.adobe.com/en/developer/commerce/storefront/setup/pre-onboarding/
• https://experienceleague.adobe.com/en/developer/commerce/storefront/setup/configuration/experience-cloud-access/
• https://experienceleague.adobe.com/en/developer/commerce/storefront/setup/configuration/cloud-manager/
• https://experienceleague.adobe.com/en/developer/commerce/storefront/setup/configuration/app-builder/
• https://experienceleague.adobe.com/en/developer/commerce/storefront/setup/configuration/api-mesh/
• https://experienceleague.adobe.com/en/developer/commerce/storefront/setup/custom-frontend/
• https://experienceleague.adobe.com/en/developer/commerce/storefront/setup/configuration/content-delivery-network/
• https://experienceleague.adobe.com/en/developer/commerce/storefront/setup/configuration/
• https://experienceleague.adobe.com/en/developer/commerce/storefront/get-started/prerequisites/

What's New highlights

whatsnew
Added new topics:

• Pre-onboarding checklist and journey alignment before deeper setup.
• Experience Cloud access Admin Console, roles, and how services connect to the storefront.
• Commerce Cloud Manager instances, environments, and IDs.
• App Builder workspace setup for events and extensibility.
• API Mesh configuration, CDN considerations, and VCL notes.
• Custom frontend React/Next.js patterns with drop-ins and SDK mounting.

Test plan

• Verify all six new pages render without build errors
• Confirm sidebar links for all new pages resolve correctly
• Check that cross-links in prerequisites.mdx and configuration/index.mdx point to the correct pages
• Review DNS section additions in content-delivery-network.mdx for accuracy
• Validate ProseFlow component renders correctly on new pages