AST-933 Add sphinx documentation

This builds sphinx documentation for a subset of C++ classes and python classes. The python documentation is not as complete as the C++ documentation, there will be a follow-up MR to fix that.

doc/build-instructions.rst

+.. _buildinstructions:
+
+Build instructions
+==================
+
+Radler can be installed as a stand-alone package, but is also installed as a part of `WSClean <https://wsclean.readthedocs.io>`_. 
+If you only want to install WSClean, it is not necessary to build Radler yourself.
+
+Dependencies
+~~~~~~~~~~~~
+Radler needs a number of dependencies in order to successfully compile. On a clean (ubuntu 22.04) system,
+the dependencies can be installed with (see also the ``docker`` directory):
+
+General packages:
+
+::
+
+    apt-get -y install wget git make cmake g++ doxygen \
+    libboost-all-dev libhdf5-dev libfftw3-dev \
+    libblas-dev liblapack-dev libgsl-dev libxml2-dev \
+    libgtkmm-3.0-dev libpython3-dev python3-distutils
+
+Astronomy-specific packages:
+
+::
+
+    apt-get -y install casacore-dev libcfitsio-dev
+
+In order to be able to build the documentation with ``make doc``, ``sphinx`` and some other documentation tools need to be installed:
+
+::
+
+    pip3 install sphinx doxygen sphinx_rtd_theme breathe myst-parser
+
+
+
+
+Quick installation guide
+~~~~~~~~~~~~~~~~~~~~~~~~
+
+::
+
+    git clone --recursive https://git.astron.nl/RD/Radler.git
+    cd Radler
+    mkdir build && cd build
+    cmake -DCMAKE_INSTALL_PREFIX=<radler_install_path> ..
+    make install
+
+
+Installation options
+~~~~~~~~~~~~~~~~~~~~
+
+(Use :code:`ccmake` or :code:`cmake -i` to configure all options.)
+
+* :code:`BUILD_WITH_PYTHON`: build Python module 'radler' to use Radler from Python
+* :code:`BUILD_TESTING`: compile tests
+
+All other build options serve development purposes only, and can/should be left at the default values by a regular user.
+
+All libraries are installed in :code:`<installpath>/lib`. The header files in
+:code:`<installpath>/include`. The Python module in
+:code:`<installpath>/lib/python{VERSION_MAJOR}.{VERSION_MINOR}/site-packages`. Make sure that your
+:code:`LD_LIBRARY_PATH` and :code:`PYTHONPATH` are set as appropiate.