Update documentation to better explain `@wip` tag

Three related changes to the documentation.

- [ ] Update the Development Tools section (`docs/source/development.rst`) to include an explanation of the `@wip` tag, and how the test data flows from cel-spec to the `features` directory tree with tags injected by the `gherkinize.py` tool.

- [ ] Add a link to this to the `README.rst` to provide a clear path to show how `@wip` tags are injected into the tests.

- [x] Add a page with the complete list of features, showing ✅ and ❌ for the features supported and the features marked `@wip`. (It's not clear precisely how to do this. It might be an useful secondary output from the `gherkinize.py` tool.) Completed by #191