Enable readthedocs build for >= 0.2a

[eirrgang]

Long term solution for documentation builds during GROMACS annual development cycle, during which public web documentation is not provided by GROMACS infrastructure, but during which we would like to provide gmxapi users with the most up-to-date easy-to-access documentation. We could add a conf.py to the gh-pages branch, in which we copy content from the gmxapi/docs Docker image, or we could maintain static content in our fork, as with #249. The solution to this issue might evolve during 2020 to get up-to-date content soonest, and add automation later.

Also, note that the structure of the GROMACS repository might change soon to support gmxapi builds as a primary use case for the repository, with setup.py as a first-class entry point to the GROMACS build system. In that case, regular readthedocs builds will become much simpler and we can use traditional RTD automation approaches.

However, note that we have not enabled the RTD web API hook on the kassonlab org account, so someone still has to go click "build" on the RTD dashboard, at least.