Readthedocs build fails

[orbeckst]

RTD CI build fails

See for example https://app.readthedocs.org/projects/distopia/builds/32113450/

Issue appears to be that the submodules are not pulled:

Error lines from RTD build 32113450

     -- Using vendored HWY subpackage
      CMake Error at libdistopia/CMakeLists.txt:27 (add_subdirectory):
        The source directory
      /home/docs/checkouts/readthedocs.org/user_builds/distopia/checkouts/193/libdistopia/highway
  
    does not contain a CMakeLists.txt file.
  
  
  CMake Error at libdistopia/CMakeLists.txt:39 (add_subdirectory):
    The source directory
  
      /home/docs/checkouts/readthedocs.org/user_builds/distopia/checkouts/193/libdistopia/googlebench
  
    does not contain a CMakeLists.txt file.
  
  
  CMake Error at libdistopia/CMakeLists.txt:58 (add_subdirectory):
    The source directory
  
      /home/docs/checkouts/readthedocs.org/user_builds/distopia/checkouts/193/libdistopia/googletest
  
    does not contain a CMakeLists.txt file.

[orbeckst]

See also #188 (comment)