Update product analytics GDK setup instructions

What does this merge request do and why?

Update product analytics setup instructions

Remove references to Snowplow, which is now the default (product_analytics_snowplow_support feature flag is no longer required)

Add note about required password to prerequisites (this has caught out reviewers a few times)

How to set up and validate locally

View the docs

Impacted categories

The following categories relate to this merge request:

• gdk-reliability - e.g. When a GDK action fails to complete.
• gdk-usability - e.g. Improvements or suggestions around how the GDK functions.
• gdk-performance - e.g. When a GDK action is slow or times out.

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.