Expand (and reduce) API documentation coverage

[adamreeve]

We now have auto-generated API documentation at https://g-research.github.io/ParquetSharp/.

This issues covers reviewing the documentation and making sure all public methods are documented.

There are also some methods where we should hide documentation as they aren't intended for use by consumers. Eg. see ColumnReader.TypedColumnReader_ReadBatch_Bool for example. There are a lot of methods like this that are protected method of a public class, but the constructors are all internal so it shouldn't be possible for users to call them and they add noise to the documentation.

These can be hidden by adding an /// <exclude /> comment (see https://dotnet.github.io/docfx/docs/dotnet-api-docs.html#filter-apis)