docs(s3-deployment): explain EFS networking requirements

[caglarkc]

Issue # (if applicable)

Closes #19777.

Description

EFS-backed deployments still need S3 connectivity. Explain the NAT and S3 gateway endpoint alternatives, the endpoint policy and subnet requirements, and memory/network considerations. Include an isolated-subnet example with an S3 gateway endpoint and an explicit memory allocation.

Validation

• The README example compiled and translated with jsii-rosetta (1 snippet, no diagnostics).
• Synthesized the example: no NAT gateways; S3 gateway endpoint route tables match the isolated deployment subnets; deployment Lambda has EFS mounted and 1024 MiB of memory.
• Checked connectivity requirements against the official Lambda, S3 VPC endpoint, and CloudFormation custom-resource response documentation.

Documentation only; no resources are provisioned automatically by this change.

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[maintainer-for-aws]

Automated review

A maintainer will still review this — treat the notes below as a starting point.

📌 Commit 10d36ac

This documentation-only change rewrites the "EFS Support" section of the aws-s3-deployment README to explain the VPC networking and memory requirements for EFS-backed BucketDeployment. It corrects earlier wording to the actual useEfs: true prop and 512 MiB ephemeral storage, explains that the deployment function must reach both source and destination buckets over HTTPS (via a NAT gateway or an S3 gateway endpoint, since the construct creates no S3 endpoint itself), documents the CloudFormation custom-resource response-bucket timeout risk, and rewrites the example to demonstrate the no-NAT path. The prose claims are accurate against the construct code and AWS behavior, and the example's identifiers and API calls resolve correctly. No issues found on this change.

🔴 0 blocking · 🟡 0 recommended · ⚪ 0 optional

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