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ARROW-9742: [Rust] [DataFusion] Improved DataFrame trait (formerly known as the Table trait)

This PR renames the `Table` trait to `DataFrame` and also contains the following improvements:

- Updates the `collect` method signature to remove the reference to an execution context (which made no sense because the table/dataframe struct was created by the context).
- Adds rustdocs for DataFrame and ExecutionContext with code examples
- Updates existing examples to call `ExecutionContext.sql()` instead of referencing logical and physical query plans - these are internal implementation details that should be of no concern to typical users
- Adds a new example that uses the DataFrame trait to build a query
- A few other minor UX improvements so that I could show some real examples

Note that the goal of this PR is mostly to document the API as it is today. There are number of design flaws that still need fixing and I will be creating follow-up PRs for those.

Closes #7972 from andygrove/dataframe

Lead-authored-by: Andy Grove <andygrove73@gmail.com>
Co-authored-by: Andy Grove <andygrove@users.noreply.github.com>
Signed-off-by: Andy Grove <andygrove73@gmail.com>