fix readme and crate metadata, add security policy for security.txt

[mooncitydev]

This PR fixes a few documentation and packaging issues:

• README: remove the incorrect Event link (there is no Event module) and link Sdk instead.
• Workspace Cargo.toml: point repository and documentation at this repo instead of ore / ore-api docs.
• Add SECURITY.md so the URL referenced in the on-chain security.txt policy field actually resolves.

No program or API logic changes.