docs(event-handler): add docs page for Bedrock Agent Function

[dreamorosi]

Summary

Changes

Please provide a summary of what's being changed

This PR adds a page to the docs dedicated to the new Event Handler for Bedrock Agent Functions.

This page borrows from the one being added in aws-powertools/powertools-lambda-python#6564 but deviates a bit due to the fact that we don't support OpenAPI specs for this feature.

This page follows the same structure as similar pages and includes:

• a mermaid diagram to explain the flow (moved the Action Group in the same "box" as the agent)
• a getting started section with SAM & CDK templates to create an agent
• a section that explains how to create a tool, how parameters are handled
• a section about error handling & custom responses
• a section about working with session attributes
• a section about how to access the current event and context

Please add the issue number below, if no issue is present the PR might get blocked and not be reviewed

Issue number: closes #3990

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[leandrodamascena]

Hey @dreamorosi! i just left some comments that probably make sense you review it.

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

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