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

demo
dolfinx
common
defines.h
dolfinx_common.h
dolfinx_doc.h
IndexMap.h
local_range.h
log.h
math.h
MPI.h
petsc.h
Scatterer.h
sort.h
Table.h
TimeLogger.h
Timer.h
timing.h
types.h
utils.h
fem
assemble_expression_impl.h
assemble_matrix_impl.h
assemble_scalar_impl.h
assemble_vector_impl.h
assembler.h	Functions supporting assembly of a finite element fem::Form
Constant.h
CoordinateElement.h
DirichletBC.h
discreteoperators.h
DofMap.h	Degree-of-freedom map representations and tools
dofmapbuilder.h
dolfinx_fem.h
ElementDofLayout.h
Expression.h
expression_evaluate.h	Evaluation of a fem::Expression, and interpolation of an Expression into a fem::Function
expression_factory.h	Factories for finite element expressions from UFCx input
FiniteElement.h
Form.h
form_factory.h	Factories for finite element forms from UFCx input
Function.h
FunctionSpace.h
functionspace_factory.h	Factories for finite element dofmaps and function spaces
integration_domains.h	Functions for computing integration domains
interpolate.h
interpolate_geometry.h	Functions for interpolating mesh geometry
kernel.h
pack.h	Functions supporting the packing of coefficient data
petsc.h
sparsitybuild.h
sparsitypattern.h	Functions for constructing finite element sparsity patterns
traits.h
utils.h	Compatibility header for fem utility functions
geometry
BoundingBoxTree.h
dolfinx_geometry.h
gjk.h
utils.h
graph
AdjacencyList.h
dolfinx_graph.h
ordering.h
partition.h
partitioners.h
sfc.h	Point partitioning by position along a space-filling curve
utils.h
io
ADIOS2Writers.h	ADIOS2-based output writers
cells.h
dolfinx_io.h
HDF5Interface.h
utils.h
vtk_utils.h
VTKFile.h
VTKHDF.h
xdmf_function.h
xdmf_mesh.h
xdmf_utils.h
XDMFFile.h
la
dolfinx_la.h
matmul.h
matrix_csr_impl.h
MatrixCSR.h
mattrans.h
petsc.h
slepc.h
SparsityPattern.h
superlu_dist.h
utils.h
Vector.h
mesh
cell_types.h
dolfinx_mesh.h
EntityMap.h
generation.h
Geometry.h
graphbuild.h
Mesh.h
MeshTags.h
permutationcomputation.h
Topology.h
topologycomputation.h
types.h	Small, foundational mesh types (enums, etc.) with minimal dependencies
utils.h	Functions supporting mesh operations
nls
dolfinx_nls.h
SNESSolver.h
refinement
dolfinx_refinement.h
interval.h
mark.h
option.h
plaza.h
refine.h
uniform.h
utils.h
dolfinx.h