Use the DataFrame How-To Guide

Use the [C# DataFrame Notebook](https://github.com/dotnet/csharp-notebooks/blob/main/machine-learning/REF-Data%20Processing%20with%20DataFrame.ipynb) to create a how-to guide in ML.NET Docs that shows how to:

- What is the DataFrame?
- Install Microsoft.Data.Analysis
- Load data (LoadCsv/LoadCsvString)
- Inspect Data (Info/Describe)
- Transform Data
- Combine Data Sources
- Merge Columns
- Merge DataFrames
- Save DataFrames