Add YAML frontmatter validation workflow

[AmePelliccia]

Add a new GitHub Actions workflow for YAML frontmatter validation.

• .github/workflows/yaml-frontmatter-validation.yml

  • Add a new workflow file named yaml-frontmatter-validation.yml.
  • Configure the workflow to trigger on push events for Markdown files (**/*.md).
  • Define a job named validate_yaml that runs on ubuntu-latest.
  • Add steps to checkout the repository, list Markdown files, and validate YAML frontmatter using the provided schema.

• .github/workflows/main.yml

  • Add a new job named YAML_Frontmatter_Validation to the existing workflow.
  • Set the job to run on ubuntu-20.04.
  • Add steps to checkout the repository and run the yaml-frontmatter-validation.yml workflow.
  • Ensure the new job runs after the Lint job.

• README.md

  • Add a section explaining the new YAML frontmatter validation workflow.
  • Provide instructions on how to use the workflow and interpret the results.
  • Include an example of a valid YAML frontmatter block.

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For more details, open the Copilot Workspace session.

[senderista]

Hello, and thanks for your interest in the Gaia project! Why are you interested in this project, and what are you trying to accomplish with this change? Why are you submitting what appears to be an entirely AI-generated change with no background or motivation?