Add robust guest YAML to TOML Markdown migration script and update migration guide

[mattstratton]

Summary

This PR adds a migration utility script (docs/utils/guest-yml-to-md.sh) to convert guest YAML data files to TOML-frontmatter Markdown files for Hugo. The script:

• Sets Title from full_name (with fallback), and generates a date field.
• Migrates all scalar and array fields (except bio, name, and full_name) to TOML frontmatter.
• Skips object fields with a warning.
• Outputs the bio field as the Markdown content.
• Is documented in docs/migration-guide.md with yq install and usage instructions.

Testing Steps

• Place guest YAML files in data/guests/.
• Run sh docs/utils/guest-yml-to-md.sh.
• Verify TOML frontmatter and content in content/guest/.

Context

• Ensures migration is robust and future-proof for new fields.
• Keeps TOML frontmatter consistent with existing guest files.

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