Add social link preview meta tags

[cybercraftsolutionsllc]

Summary

• replace the bare default description with share-friendly Sphinx Community copy
• add Open Graph title, description, image, URL, and site metadata
• add Twitter card title, description, and image metadata

Fixes stakwork/sphinx-tribes#670

Validation

• git diff --check
• PowerShell check that og:title, og:description, og:image, twitter:card, and twitter:image are present in public/index.html

Notes

• This is a static metadata update to the CRA HTML template, so no app bundle behavior changed.