Host the Python docs on ReadTheDocs

From time to time the discussion arise about moving docs.python.org to readthedocs, there's even an experimental python.readthedocs.io.

I think there's many pros and cons of using readthedocs.

I'd personally be in favor of it if we (the PSF) support them (the cpython Doc is a big one with a lot of traffic, it take around 24h of CPU to build all versions × languages with all PDF A4, PDF letter, HTML, plaintext, epub).

In the other hand it's not an easy task, among other things docs.python.org is not only about generating the docs but also hosting history:

• https://docs.python.org/release/ (this is all the tagged commits in the cpython repo)
• https://docs.python.org/2.0/ (yes we keep old versions, and I'm not willing to see this disapear)

Empieza revisando el alojamiento y el proceso de compilación actuales de docs.python.org y compáralos después con la configuración experimental de python.readthedocs.io. Presta especial atención al historial de versiones, Python 2.0 y otras versiones antiguas, así como a la carga de compilación indicada para múltiples versiones, idiomas y formatos. Se considerará terminado cuando exista un plan de migración concreto que preserve esos requisitos y resuelva las disyuntivas entre la PSF y el alojamiento.