Attach docs before publishing immutable releases

[lukewilliamboswell]

Summary

• pin publish-release to roc-lang/release-package commit 48ccc909bd8f6ba60feb1d33d8b6ac1dc1c7af36
• pass docs.tar.gz through the new additional_assets input
• remove the post-publication asset upload that fails for immutable releases

The release-package change is under review in roc-lang/release-package#3. Its CI passes and the action is pinned to the exact commit.

Validation

• release-package CI passed
• release workflow YAML parses successfully
• git diff --check