Add doc_gen validation

[DavidSouther]

Add a GitHub Actions workflow to run the https://github.com/awsdocs/aws-doc-sdk-examples-tools validation scripts.

Description

This workflow will check .doc_gen folder contents for

Testing

act --job validate-doc-metadata for main changes; tests in -tools project for details.

Types of changes

• [ ] Bug fix (non-breaking change which fixes an issue)
• [X] New feature (non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)

Checklist

We require a second engineer to validate the PR before merging

• [ ] My code builds in Gamma and passes backward compatibility validation (required)
• [X] My code follows the code style of this project
• [ ] My change requires a change to the documentation
• [X] I have updated the documentation accordingly
• [X] I have read the README document
• [X] I have added tests to cover my changes
• [X] All new and existing tests passed

New/existing dependencies impact assessment, if applicable

GitHub Actions dependency on https://github.com/awsdocs/aws-doc-sdk-examples-tools

Note to reviewers: Please follow runbook to update the internal open source attribution tool

License

• [X] I confirm that this pull request can be released under the Apache 2 license

[jnunn-aws]

This file has been added to the repo out-of-band from this PR.