Fix contributor avatars and docs: use avatar key and correct anomaly image extension

[bpmct]

Summary

• Switch profile frontmatter key from avatar_url to avatar for three contributor namespaces
• Correct anomaly namespace to reference existing avatar.jpeg
• Update documentation to match (avatar key) in MAINTAINER.md and CONTRIBUTING.md

Files updated

• registry/anomaly/README.md (avatar_url -> avatar, .png -> .jpeg)
• registry/sharkymark/README.md (avatar_url -> avatar)
• registry/ericpaulsen/README.md (avatar_url -> avatar)
• MAINTAINER.md (example frontmatter: avatar_url -> avatar)
• CONTRIBUTING.md (sample frontmatter and note: avatar_url -> avatar)

Why

• The site/validator expects avatar (not avatar_url) for contributor profiles
• Docs previously suggested avatar_url, which led to a few broken avatars

Verification

• Confirmed image files exist at the referenced paths:
  • anomaly: ./.images/avatar.jpeg
  • sharkymark: ./.images/avatar.png
  • ericpaulsen: ./.images/avatar.png
• These changes will render contributor avatars correctly on the next deploy

Test plan

• Build registry-server site and verify contributor avatars render on contributor list and profile pages for Jay, Mark, and Eric

Co-authored-by: bpmct 22407953+bpmct@users.noreply.github.com

[matifali]

Good catch, Ben.

[blink-so]

Thanks!