Add a GitHub social preview image for ParseOtter

[showgp]

Problem

ParseOtter has screenshots and branding assets, but repository link previews still need a stronger GitHub social preview image.

Proposed Work

• Create a 1280x640 social preview image using existing ParseOtter branding.
• Communicate the core value: PDF/EPUB to Markdown, open source, self-hostable.
• Keep the design readable at small preview sizes.
• Avoid private deployment details, screenshots with sensitive data, or claims that depend on changing metrics.

Acceptance Criteria

• A 1280x640 image asset exists or is ready for upload by a maintainer.
• The image uses ParseOtter branding consistently.
• The image text remains legible in GitHub/social previews.
• Maintainer can upload it in repository settings.