fix: add a2a card to docs

[Sam Crowder (samecrowder)]

Overview

Customer that tried out the /a2a endpoint asked where to find the agent card. It was previously unfindable, so this makes things more clear.

Type of change

Type: Update existing documentation

Checklist

• I have read the contributing guidelines
• I have tested my changes locally using docs dev
• All code examples have been tested and work correctly
• I have used root relative paths for internal links
• I have updated navigation in src/docs.json if needed
• I have gotten approval from the relevant reviewers
• (Internal team members only / optional) I have created a preview deployment using the Create Preview Branch workflow

[Copilot]

Pull Request Overview

This PR adds documentation for A2A Agent Card discovery to help customers understand how to find and retrieve agent cards for A2A-compatible assistants. Previously, customers couldn't find where to access the agent card information needed for agent-to-agent communication.

• Added a new "Agent Card Discovery" section explaining how to retrieve agent cards
• Documented the GET endpoint for accessing agent card information
• Clarified what information is included in the agent card

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[github-actions]

Preview ID generated: preview-samadd-1757969204-7735ab6