Feature: Add site-wide social_image param for Open Graph sharing fallback

[mattstratton]

Summary

• Adds a new [params] social_image parameter to the site config, used as the default Open Graph/Twitter sharing image for any page that does not have images set in its frontmatter (including the homepage).
• Updates all archetypes to include an images field for social sharing, ensuring new content types are set up for best practices.
• Updates documentation in REFERENCE.md to describe the new parameter and its usage.
• Updates the SEO partial to use this fallback logic for all pages.

Testing Steps

• Set social_image in your site params and verify it appears as the sharing image for pages without a specific images frontmatter value (e.g., About, homepage).
• Confirm that pages with images in frontmatter still use their own image.

Context

Fixes #172
Fixes #187

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