docs(core): document special construct IDs

[ZedingZhang]

Issue # (if applicable)

Closes #21410.

Reason for this change

The special treatment of construct IDs named Default and Resource during CloudFormation logical ID generation was only documented in the implementation. Users could unintentionally change logical IDs or produce duplicate logical IDs without knowing that these names have special behavior.

Description of changes

Adds a new section to the aws-cdk-lib README that documents:

• How construct paths are used to generate CloudFormation logical IDs.
• The replacement risks associated with renaming or moving constructs.
• How Default is removed entirely from logical ID calculation, including the hash.
• How Resource is omitted from the human-readable portion of nested resource logical IDs while remaining part of the hash.
• The intended uses and collision risks of both special IDs.

This is a documentation-only change and does not modify logical ID behavior.

Describe any new or updated permissions being added

None.

Description of how you validated changes

• Ran git diff --check.
• Ran MarkdownLint against packages/aws-cdk-lib/README.md using the repository's MarkdownLint configuration.
• Compared the documentation against the implementation in core/lib/private/uniqueid.ts and the existing logical ID tests.

A full build was not run because the change only adds prose and a Markdown table, with no code or Rosetta examples.

Checklist

• My code adheres to the CONTRIBUTING GUIDE and DESIGN GUIDELINES

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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