message: [src/doc/book] Update SUMMARY.md
diff: * [Working on an existing Cargo project](02-03-working-on-an-existing-project.md)
  * [Dependencies](02-04-dependencies.md)
  * [Project layout](02-05-project-layout.md)
  - * [Tests](02-06-tests.md)
  - * [Continuous Integration](02-07-continuous-integration.md)
  + * [Cargo.toml vs Cargo.lock](02-06-cargo-toml-vs-cargo-lock.md)
  + * [Tests](02-07-tests.md)
  + * [Continuous Integration](02-08-continuous-integration.md)
  * [Cargo In Depth](cargo-in-depth.md)
  * [Specifying Dependencies](03-01-specifying-dependencies.md)
  * [Cargo.toml Format](03-02-manifest.md)
  