Add .readthedocs.yml for configuring Read the docs build

Created by: lgp171188

This PR fixes the failing Read the docs build for Ocim documentation (hosted at https://ocim.opencraft.com) by adding a .readthedocs.yml configuration file which explicitly configures the build.

Without this PR, RTD was trying to install the requirements from the top-level requirements file and failing because the MySQL client library required a MySQL client package to be installed. This is now fixed by configuring RTD to install only the requirements file under the documentation directory,

Testing instructions:

1. Check and verify that https://ocim.opencraft.com/en/guruprasad-bb-3528-fix-readthedocs-builds/ has all the documentation present in the documentation/ directory of this repository.
2. Compare with the documentation at https://ocim.opencraft.com/ which has some of the recent documentation changes missing due to the build failures. For example, a new section about the reset_db management command was added to the master branch a while ago but doesn't show up under https://ocim.opencraft.com but shows up in the build branch for this PR.