Q. Is there a tool to generate documentation of my Azure resources?
A. It is possible to export out the JSON that makes up the content of each Resource Group in your Azure subscription which provides basic documentation and can even be visualized with http://armviz.io/ however depending on the number of resource groups you have and the level of documentation you require this will likely not meet requirements.
I could not find a free tool however there is a paid tool at https://www.azuredockit.com/ which generates comprehensive documentation. There is an example on the site of the type of materials generated.