I finally found the issues. Firstly, I had to add all the dependent packages in the requirements.txt in the docs directory. In my case I had the following

mpi4py
colossus
numpy
scipy
sphinx
sphinx_rtd_theme
numpydoc
sphinx-autoapi

Next, under the python section and install subsection, add

- method: pip
  path: .

If it weren't for mpi4py, everything would work fine. However, since mpi4py needs additional libraries installed, we need to add

apt_packages:
  - libopenmpi-dev

under the build section of .readthedocs.yaml file.