Add a read the docs

[jahav]

A testing PR to add a read-the-docs to the repo.

• ClosedXML needs a better documentation,
• ever PR should include docs about feature/changes => need docs in the repo .
• Github markdown is very limited
• Github wiki doesn't do versioning by release

[jahav]

For future reference

• Sphix is a RST/MkDoc to other formats generator (in our case RST to HTML)
• Breathe-doc is a generator that takes XML from Doxygen and converts it to csharp documentation. Breathe is in a "minimal active work" mode (source)
• sphinx_csharp is a domain extension for Breathe-doc to generate csharp elements (classes, methods, structs), e.g. parses method signature and outputs nodes to write to RST.
• The Doxygen generation must be done through Python subprocess.call in conf.py, otherwise ClosedXML would need to store XML files in repo (not acceptable). See Running on Read the Docs - Generating Doxygen XML Files
• Breathe-doc added support for C# in this PR C# Support breathe-doc/breathe#550
• Breathe-doc uses an extensions to generate the markup from Doxygen XML. Extension must be registered in the Sphinx.
• There are several extensions, a dead one (has a obsolete package on PyPI that throws errors), maintained fork that doesn't have a package on PyPI, a not-so-pretty one where I don't know how well it is integrated to Breathe.
• Breathe ::member:: option shows even public methods from non-public methods. That kind of make sense, why should non-public class have public methods, but not something we want to show. EXTRACT_PRIVATE in Doxygen should have stopped it, but doesn't... not really a problem.

conf.py

# Need to have breathe to generate xml to rst and sphinx_csharp (note the underscore) 
# to generate pretty Doxygen documentation
extensions = ['sphinx_csharp','breathe']

import subprocess, os

read_the_docs_build = os.environ.get('READTHEDOCS', None) == 'True'

if read_the_docs_build:
     subprocess.call('doxygen', shell=True)
     subprocess.call('pip install git+https://github.com/rogerbarton/sphinx-csharp.git', shell=True)