docs: add a guide for Storefront analytics with Google Analytics 4

[Björn Meyer (BrocksiNet)]

Summary

• Adds the guide "Storefront analytics with Google Analytics 4" to the advanced Storefront guides.
• It documents the GA4 payload of the Storefront, the delivery through gtag or the Google Tag Manager data layer, and the list IDs that Tag Manager triggers can rely on.
• It shows how extensions make their own products and lists visible to analytics, and how to report custom events.
• The release notes of the Shopware stack link to this page instead of repeating the details.

Related links

• fix(storefront): send GA4 ecommerce events in the documented format shopware#21606 to feat(storefront): report select_item and the list of a product to analytics shopware#21612, the stacked GA4 PRs this guide documents
• chore(storefront): combine the GA4 stack for a preview environment shopware#21613, the combined preview of the stack

Checklist

• I reviewed affected links, code samples, and cross-references, including PageRef references where relevant.
• I added or updated redirects in .gitbook.yaml if pages were moved, renamed, or deleted.
• I updated .wordlist.txt (and sorted it) if spellcheck flags new legitimate terms.
• Any required dependent changes in downstream modules have already been merged and published.
• This pull request is ready for review.

Notes

This is a draft until the Shopware stack is merged. The guide states Shopware 6.7.16.0 as the first version with this behavior, which has to match the release the stack lands in.

[shopware-dev-docs-connector]

Developer Docs healthcheck

Status: Completed with success.
Repository: shopware/docs
Commit: 2ae3e64
Preview: https://developer-documentation-2q8m1actv-shopware-frontends.vercel.app
Workflow run: #5665

[Björn Meyer (BrocksiNet)]

Manual test plan for the GA4 stack: https://gist.github.com/BrocksiNet/8aec805bd315dfd7ab4bb0ab86726c4f

It covers the whole stack (shopware/shopware#21606 to shopware/shopware#21612). For each area there are test cases with steps and the events and payloads to expect, plus console snippets that show the events and the deployed version. The plan also lists the findings from the test runs and which PR fixes each of them.

Every case that can be tested was run in a browser, on a local trunk instance and on the preview environment of shopware/shopware#21613 (preview head shopware/shopware@e1769598880). All findings are fixed. Cases that cannot be tested on the preview instance are listed with the reason.