Documentation Update: Need X-Ray VPC endpoint for private VPC deployment mode

[gauharbains]

Description of changes:
This PR updates the deployment documentation for deployments in private VPC mode where AgentCore Runtime runs in private subnets without internet access. The OpenTelemetry instrumentation (aws-opentelemetry-distro) is configured on all agent patterns and exports traces to AWS X-Ray. Without the com.amazonaws.{region}.xray VPC interface endpoint reachable from the runtime's subnets, trace export silently fails — the BatchSpanProcessor retries every 5 seconds and never succeeds.

[harshitkgupta]

How changing the deployment.md updates the VPC?

[gauharbains]

How changing the deployment.md updates the VPC? @harshitkgupta

This documentation change is for private VPC deployments, where a user wants to deploy the runtime in their own VPC. The VPC must have certain endpoints configured. This update adds documentation for an additional required endpoint.

[kaleko]

@harshitkgupta for clarification, the way FAST supports VPC deployments is:

1. Provide instructions for the users to set up their own VPC or configure their existing VPC (FAST does not create any VPCs)
2. User provides VPC information (e.g. ARN) in the config.yaml
3. FAST deploys runtime into the existing VPC

This PR just improves the documentation for step # 1