docs: add missing crate readmes

[giwaov]

Summary

• Add README files for crates under code/crates that did not have one
• Keep the existing core-consensus and discovery READMEs unchanged
• Use short package-specific summaries based on each crate's current purpose

Closes #710

Verification

• Confirmed every directory under code/crates now has a README.md`n- git diff --cached --check passed before commit

Contribution eligibility

• Issue opened by a repository contributor
• No open PR found for code: Each crate should have its own README #710 before starting

[github-actions]

Hi @giwaov,

Thank you for your interest in contributing to Malachite.

This PR has been automatically closed because you are not assigned to issue #710. We require contributors to be explicitly assigned to an issue before submitting a PR.

To contribute properly:

1. Comment on issue code: Each crate should have its own README #710 requesting assignment
2. Wait for maintainer approval
3. Only submit a PR after you have been assigned

Please see our CONTRIBUTING.md for more details.