docs: use registerConfig in existing resource guide

[ikenyal]

Problem

The existing-resource guide teaches handler.addEnvironment() for resource identifiers. This conflicts with the current API design guidance, which requires registerConfig() for resource mappings to avoid Lambda environment-variable capacity limits.

Issue #, if available: N/A

Changes

• Replace runtime resource-configuration examples with registerConfig().
• Explain that registered values remain available through process.env before the backend module loads.
• Keep direct handler use for IAM grants unchanged.

Validation

• Ran npx --yes markdown-link-check docs/guides/extending-with-existing-aws-resources.md.
• Ran git diff --check.
• Confirmed the examples against the current registerConfig() implementation and Lambda initialization order.

No automated tests were added because this is a documentation-only change.

Checklist

• PR description included
• Tests are changed or added
• Relevant documentation is changed or added (and PR referenced)

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By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

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