amp-story-auto-analytics documentation update

We need to add a section linking to https://support.google.com/analytics/answer/10075209?hl=en#zippy= which describes how to view custom dimensions and metrics.

## 📖 Missing or Out-of-Date Documentation

### Problem
A clear and concise description of what content is missing or needs updating. It could be a missing guide explaining a concept, a step-by-step tutorial, or something that is confusing or didn't work when you tried it.

### Location
Section of the site where the content lives (or should).

### Content type
Such as tutorial, an updated paragraph, or reference docs?

### Information architecture (optional)
If you have ideas on how we should structure the content, tell us!