feat: generate contributors PNG for README

[skyash-dev]

Resolves #8237

What this does

This PR adds a generated contributors image to the README to improve visibility for project contributors without reintroducing a large table.

Implementation details

• Generates a single static contributors.png at build time from .all-contributorsrc
• Uses a GitHub Actions workflow that runs only when the contributors list changes (.all-contributorsrc)
• The README embeds the generated PNG (single request, no runtime network access)

Why PNG

I explored both SVG and PNG approaches:

• GitHub does not render SVGs that reference external images
• Base64-embedded SVGs do work, but in practice resulted in a much larger artifact (~56 MB vs ~3 MB for PNG)
• Given file size as a priority and the fact that this is a generated visual artifact, a single PNG felt like the more predictable trade-off

Notes

• Avatars are resized to match the display size to keep the output reasonable
• The workflow runs in CI only and has no impact on runtime performance

Happy to iterate if there’s any preference.