Feature: Add docs on working with multi-index DataFrames

[remeika]

For a users with rudimentary Pandas experience (like me), it takes quite a while to discover that:

1. wbdata returns DataFrames with multiple indices
2. How to perform simple operations, like select a few rows, from a multi-index DataFrame

Adding docs on this will provide a much better quickstart experience for new users.

NB: I plan on adding these docs.