Improve overview of online documentation with autosummary

[jorgensd]

Expose autosummary for all variables: https://www.sphinx-doc.org/en/master/usage/extensions/autosummary.html#confval-autosummary_ignore_module_all

[francesco-ballarin]

(Partially OT): except for maybe dolfinx/__init__.py, do we need __all__ in the other submodules? It gets populated by default with everything that gets imported in the __init__.py file.

[jorgensd]

(Partially OT): except for maybe dolfinx/__init__.py, do we need __all__ in the other submodules? It gets populated by default with everything that gets imported in the __init__.py file.

We need __all__ for autosummary to work.

[jorgensd]

There are also functions that we import that we do not want to expose (such as helper functions, external imports etc).