[Task] Add a neutral TrustDB, immudb, Rekor, and Trillian comparison

[ryan-wong-coder]

Background\n\nUsers often confuse immutable databases, transparency logs, software-supply-chain logs, and portable offline evidence. A neutral comparison can explain when TrustDB is appropriate and when another project is a better fit.\n\n## Scope\n\nAdd a concise comparison document based on current public project documentation.\n\n## Acceptance criteria\n\n- Compares primary use case, data model, verification model, portability, hosting model, and ecosystem focus.\n- Cites primary project documentation.\n- Avoids unsupported performance or security superiority claims.\n- Includes a section explaining complementary deployment patterns.\n- Is linked from the README or website documentation.