Update ssot of AI features in GitLab documentation

Status

Closed in favor of !153741 (merged)

What does this MR do and why?

This MR updates the GitLab Documentation page that lists our AI features. I believe some of these features have been deprecated, merged, or have evolved and this page is not accurate anymore.

I would like this list to represent all of our AI features.

This MR:

• rewrites the Duo Chat entry to be more representative of its current functionality
• removes the test generation entry from the table as this functionality has been absorbed into the chat

Next steps

There are more places that need to be updated:

• https://handbook.gitlab.com/handbook/engineering/development/data-science/ai-powered/#features
• https://about.gitlab.com/gitlab-duo/
• https://docs.gitlab.com/ee/user/ai_features.html#language-models
• Where else?

Also:

• Visualization graphs feature will be added in separate mr
• The marketing names do not match the way the feature is described in the product. (E.g. on our docs "Summarize MR", versus in the product "Summarize code changes"). This was an intentional decision, but we may want to revisit at some point.

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