docs: update README banner

[cyfyifanchen]

Summary

• Replace the README hero banner with the new Raven GitHub banner.
• Reference the compressed image through GitHub User Content so no image binary is added to the repository.
• Preserve the original 2089 x 753 dimensions while reducing the file from 1,144,689 bytes to 352,115 bytes.

Type

• Fix
• Feature
• Docs
• CI / tooling
• Refactor
• Other

Verification

• Relevant tests pass locally
• Relevant lint / type checks pass locally
• User-facing docs or screenshots are updated when needed

Commands:

• git diff --check origin/main...HEAD - passed
• make check-large-files - passed
• git merge-tree --write-tree HEAD origin/main - passed with no conflicts

Risk

• Security impact considered
• Backward compatibility considered
• Rollback path is clear for risky changes

This is a low-risk documentation-only change. Rollback requires reverting the README image URL.

Related Issues

N/A