Consolidate Duo Agent Platform docs into /howto/ai

Overview

As @jessieay suggested here, we should merge doc/howto/duo_agent_platform.md and doc/howto/ai/_index.md since having both pages is confusing.

Impacted categories

The following categories relate to this issue:

• 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.