Update Pandas DataFrame Reader Example To Use Built-In Reader

[mkierzenka]

The Use of Custom Readers that support Pandas DataFrames example is helpful to learn about how to implement a custom data reader in Stone Soup, but I think it may be confusing to new users. When I first read it, I thought it implied that in order to add support for reading truth/measurement data from DataFrames I needed to write my own _DataFrameReader, DataFrameGroundTruthReader, and DataFrameDetectionReader classes as shown in the first section of that example. However, these classes are now natively included in Stone Soup: stonesoup/reader/pandas_reader.py.

Perhaps this example should be updated to remove those sections showing reimplemented code and instead utilize the built-in classes? What do you think?

[apiszcz]

FYSA: #971