docs: add Duo usage API GraphQL docs

Relates to issue #583356

Changes

This MR adds comprehensive documentation for the instance-level Duo usage API endpoint to doc/api/graphql/duo_and_sdlc_trends.md.

New Documentation Section: "Retrieve instance-level AI usage data"

• Hugo details block indicating availability for GitLab Self-Managed and GitLab Dedicated
• History block documenting introduction in GitLab 18.7 with experiment status
• Clear description explaining the endpoint is for administrators to retrieve AI usage data
• Complete GraphQL query example with all available parameters
• Example response showing real event data structure

Authorization Requirements Subsection

Documents that the endpoint requires administrator-level access

Filtering and Pagination Subsection

Comprehensive documentation of query parameters:

• startDate - defaults to 7 days before current date
• endDate - defaults to current day
• events array - for filtering by specific event types
• userIds array - for filtering by specific users
• Standard GraphQL pagination support

Data Retention and Sources Subsection

Explains storage mechanisms:

• PostgreSQL storage with 3-month retention period
• ClickHouse storage with indefinite retention when configured

Reference to Tracked Events

Includes reference to ai_tracking.rb for viewing all tracked events

The documentation follows existing patterns and structure in the file, ensuring consistency with the group/project-level endpoint documentation.