Add documentation for new features

[ollietulloch]

ndr_import has had a number of changes that do not have appropriate documentation in docs/, below is a list of mapping funtionality that requires documentation:

• Adding optional last_data_column in NdrImport::Table mappings (Allow optional final column to be defined in the NdrImport::Table mapping #61)
• Conditionally preserve blank lines when joining lines in non-tabular data. (Preserve blank lines when joining lines in non-tabular data #41)
• Password protected files (Feature/file password #40)
• NdrImport::PdfForm::Table mappings (# Added ability to extract and transform PDF form data #36)
• Generate "per row" identifiers (ndr_import table can now automatically generating a per row identifier #34)
• Add validations to field mappings (Add validations to field mappings #27)
• Liberal parsing of delimited data (Liberal parsing #24)
• NdrImport::Xml::Table mappings (Xml file support #22)
• Allow any delimiter to be specified in an NdrImport::Table (Allow any delimiter to be specified in an NdrImport::Table #18)
• NdrImport::FixedWidth::Table mappings (# Add fixed width functionality #17)
• NdrImport::Avro::Table and NdrImport::File::Avro (Adds avro file support #94)