Make README generation local to this repository

[ulises-jeremias]

Summary

Remove the dependency on nanlabs/awesome-nan for README generation and make the workflow self-contained.

Context

The current generate-readme.yml workflow checks out nanlabs/awesome-nan and runs awesome-nan/tools/readme-generator/main.ts. Public repositories should own their CI dependencies where practical so contributors can understand and update them locally.

Acceptance Criteria

• README generation no longer checks out nanlabs/awesome-nan.
• The required generator logic is local to this repository or replaced by a documented local command.
• README.md.tmpl and examples.json continue to generate the same README content unless intentional changes are documented.
• The workflow uses maintained GitHub Actions versions.
• Documentation explains how contributors can run README generation locally.

Suggested Validation

• Run the README generation command locally.
• Confirm README.md has no unexpected diff.
• Parse the workflow YAML.