Update documentation to better explain `@wip` tag

[slott56]

Three related changes to the documentation.

1. 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.

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

3. 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.)