Add a "scope" document

[TomAugspurger]

e.g. https://numpy.org/neps/scope.html

Some random thoughts

1. Focus on "user API for working with tabular data" (de-emphasize things like in-memory format, array library, algorithms)
2. Focus on "spirit" rather than strict rules.
3. Single-machine (but not necessarily single-core?)
4. In-memory only?
5. Visualization?
6. What readers / writers are in-scope?
7. What future extension types are in scope?

[jbrockmendel]

Discussed a couple of dev meetings back. Consensus was this was poorly defined and a moving target. Closing as no-action.