Do not use mpi_cmake_modules

[luator]

From what I saw, this package only contains high level documentation and nothing actual API-related. Therefore, I suggest to not use mpi_cmake_modules for building the documentation but instead adding a basic Sphinx configuration.

Pros:

• Fewer dependencies (basically only Sphinx and maybe some plugins).
• Higher flexibility. We can, for example, manually define the order of pages (independent of the name) and add nesting.
• It's easy to set up an automated build directly on GitHub (using the same setup as on mw_dual_motor_torque_ctrl).

Cons:

• I don't see any, but maybe I am missing something?

@vincentberenz Let me know what you think. If you agree, I'll take care of the conversion.

[vincentberenz]

@luator

I totally agree

[luator]

Done in #6