[Release] Prepare for v0.3.1

[ManaSugi]

RELEASE: libseccomp: v0.3.1

TODO:

• [ ] Solve all PRs and issues in https://github.com/libseccomp-rs/libseccomp-rs/milestone/4
• [ ] Validate the crate documentation
  • [ ] Run make doc and verify the documents on your local environment
• [ ] Change Cargo.toml, README.md, and CHANGELOG.md for v.0.3.1 on release/v0.3.x branch
• [ ] Create v0.3.1 tag
• [ ] Publish the libseccomp crate on crates.io
• [ ] Create the release page in GitHub