As tested in #649, the readthedocs environment does not run out of memory anymore when building the conda environment based on the default env_climada.yml specs, see the corresponding build logs. Installing the dependencies just takes a bit longer, some 320 sec as opposed to 250 sec. This gives us the opportunity to remove the requirements/env_docs.yml altogether, which currently causes a considerable maintenance overhead. The downside is that we can only specify a single environment file for readthedocs. This means that the dependencies for building the documentation have to be moved to the base image specs in env_climada.yml.