Here is a list of all documented files with brief descriptions:

gridformat
common
field.hpp	Abstract interface for fields of values that is used by writers/readers to store fields.
compression
common.hpp	Common classes used in the context of data compression
concepts.hpp	Concepts related to data compression
decompress.hpp	Decompress compressed data
lz4.hpp	Compressor using the LZ4 library
lzma.hpp	Compressor using the LZMA library
zlib.hpp	Compressor using the ZLIB library
encoding
ascii.hpp	Encoder and stream using ascii
base64.hpp	Encoder and stream using base64
concepts.hpp	Concepts related to data encoding
encoded_field.hpp	Wraps a field and makes it streamable using encoding
raw.hpp	Encoder and stream for raw binary output
grid
concepts.hpp	Grid concepts
converter.hpp	Converter between grid formats
image_grid.hpp	Predefined image grid implementation
reader.hpp	Base class for grid data readers
writer.hpp	Base classes for grid data writers
traits
cgal.hpp	Traits specializations for cgal triangulations in 2D 3D
dealii.hpp	Traits specializations for dealii triangulations
dolfinx.hpp	Traits specializations for dolfinx meshes
dune.hpp	Traits specializations for dune grid views
mfem.hpp	Traits specializations for mfem::Mesh
vtk
appendix.hpp	Helper classes for writing VTK appendices of xml formats
attributes.hpp	Helper functions to get the VTK-specific names of things
common.hpp	Common functionality for VTK writers
data_array.hpp	Wraps a field and exposes it as VTK data array. Essentially, this implements the operator<< to stream the field data in the way that VTK file formats require it.
hdf_common.hpp	Common functionality for writing VTK HDF files
hdf_image_grid_reader.hpp	Reader for the VTK HDF file format for image grids
hdf_image_grid_writer.hpp	Writer for the VTK HDF file format for image grids
hdf_reader.hpp	Readers for the VTK HDF file formats
hdf_unstructured_grid_reader.hpp	Reader for the VTK HDF file format for unstructured grids
hdf_unstructured_grid_writer.hpp	Writer for the VTK HDF file format for unstructured grids
hdf_writer.hpp	Writers for the VTK HDF file formats
parallel.hpp	Helper function for writing parallel VTK files
pvd_reader.hpp	Reader for .pvd time series file format.
pvd_writer.hpp	Writer for .pvd time-series file format.
pvti_reader.hpp	Reader for .pvtp file format.
pvti_writer.hpp	Writer for parallel .pvti files.
pvtp_reader.hpp	Reader for .pvtp file format.
pvtp_writer.hpp	Writer for parallel .pvtu files.
pvtr_reader.hpp	Reader for .pvtr file format.
pvtr_writer.hpp	Writer for parallel .pvtr files.
pvts_reader.hpp	Reader for .pvts file format.
pvts_writer.hpp	Writer for parallel .pvts files.
pvtu_reader.hpp	Reader for .pvtu file format.
pvtu_writer.hpp	Writer for parallel .pvtu files.
pxml_reader.hpp	Base class for readers of parallel vtk-xml file formats.
vti_reader.hpp	Reader for .vti file format.
vti_writer.hpp	Writer for .vti file format.
vtp_reader.hpp	Reader for .vtp file format.
vtp_writer.hpp	Writer for .vtu file format.
vtr_reader.hpp	Reader for .vtr file format.
vtr_writer.hpp	Writer for .vtr file format.
vts_reader.hpp	Reader for .vts file format.
vts_writer.hpp	Writer for .vts file format.
vtu_reader.hpp	Reader for .vtu file format.
vtu_writer.hpp	Writer for .vtu file format.
xml.hpp	Helper classes and functions for VTK XML-type file format writers & readers
xml_time_series_writer.hpp	Writer for time series of a VTK-XML file format. Populates the "TimeValue" metadata field supported by VTK.
gridformat.hpp	This file is the entrypoint to the high-level API exposing all provided readers/writers through a unified interface
reader.hpp	A generic reader providing access to the readers for all supported formats
writer.hpp	A generic writer providing access to the writers for all supported formats