Documentation: include submodules on readthedocs

Anton Joubertrequested to merge
fix-doc-build into develop

Build starting failing on readthedocs with:

  CMake Error at CMakeLists.txt:30 (find_package):
    By not providing "FindTango.cmake" in CMAKE_MODULE_PATH this project has
    asked CMake to find a package configuration file provided by "Tango", but
    CMake did not find one.

FindTango.cmake is provided by the git submodule: TangoCMakeModules.

This branch worked on readthedocs:

image

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