Add custom getitem to cudf.pandas

[galipremsagar]

Description

This PR improves the getitem times by avoiding a lot of the cudf.pandas machinery if it's a simple scalar getitem.

# on branch-25.02:

In [1]: %load_ext cudf.pandas

In [2]: import pandas as pd

In [3]: s = pd.Series([1, 2, None, 5, 3])

In [4]: %timeit s[0]
3.32 ms ± 37.1 μs per loop (mean ± std. dev. of 7 runs, 100 loops each)


# This PR:

In [1]: %load_ext cudf.pandas

In [2]: import pandas as pd

In [3]: s = pd.Series([1, 2, None, 5, 3])

In [4]: %timeit s[0]
3.02 μs ± 19.9 ns per loop (mean ± std. dev. of 7 runs, 100,000 loops each)

```

## Checklist
- [x] I am familiar with the [Contributing Guidelines](https://github.com/rapidsai/cudf/blob/HEAD/CONTRIBUTING.md).
- [x] New or existing tests cover these changes.
- [x] The documentation is up to date with these changes.

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[galipremsagar]

/okay to test

[galipremsagar]

/okay to test

[galipremsagar]

/okay to test

[galipremsagar]

/okay to test

[galipremsagar]

/okay to test