[fabricebrito]

Expand the repository's brief testing documentation into a practical guide for running, understanding, and extending the existing water-bodies suite.

• Expand the README with repository structure, test coverage, setup instructions, CI behavior, and contribution guidance.
• Add documentation pages for setup, the example workflow, test authoring, fixture reproducibility, CI, and troubleshooting, with workflow diagrams and linked navigation.
• Clarify the limits of flexible file assertions and deterministic checksums, and remove missing asset references and a duplicate extension entry from the MkDocs configuration.

Validation: mkdocs build --strict passed, along with checks for local links, navigation targets, code fences, and git diff --check. The CWL processing suite was not rerun because these changes only affect documentation and site configuration.