Mention new FF in product analytics instructions

What does this merge request do and why?

Updates the product analytics documentation, mentioning

1. New product_analytics_beta_optin feature flag
2. Experiment features and product analytics checkbox in root group settings

Merge request checklist

• This change is backward compatible. If not, please include steps to communicate to our users.
• Tests added for new functionality. If not, please raise an issue to follow-up.
• Documentation added/updated, if needed.
• Announcement added, if change is notable.
• gdk doctor test added, if needed.
• Add the ~highlight label if this MR should be included in the CHANGELOG.md.