Add documentation for analytics natural language querying experiment

What does this MR do and why?

Adds user docs for the product analytics natural language querying experiment.

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Screenshots or screen recordings

Before	After
	(Added language model detail)

How to set up and validate locally

Apply setup steps for backend MR this one depends on:

• Enable AI features locally: https://docs.gitlab.com/ee/development/ai_features/#test-ai-features-locally
• Make sure you've enabled Vertex access and configured it with your GDK. This action does not use Anthropic's API, only Vertex.
• Enable the :generate_cube_query flag locally.

Then:

1. Follow these instructions to setup Product Analytics in GDK.
2. Onboard Product Analytics by setting up the feature at Project > Analytics > Analytics dashboards.
3. Visit the visualization designer at Project > Analytics > Analytics dashboards > Visualization designer.
4. Verify new Learn more. link works as expected

Then:

1. Verify docs changes:
2. http://gdk.test:3005/ee/user/ai_features.html#language-models
3. http://gdk.test:3005/ee/user/analytics/analytics_dashboards.html#generate-a-custom-visualization-with-gitlab-duo

Related to #441564 (closed)