Use tudatpy conda package to build docs on Read the Docs

[niketagrawal]

This is a follow up to the changes introduced via #157.

After pull request #157, tudat-multidoc is no longer needed to build the API documentation locally, however, read the docs continues to use multidoc to build the docs as evident from the body of the if loop in docs/src/conf.py.

Modify the steps to build the docs on Read the Docs to use tudatpy conda package instead of multidoc. Point to the location where the binaries are stored, i.e., the location of the installation of tudatpy conda package on the Read the Docs server.

[niketagrawal]

Fixed with PR #175