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)

先审查当前 docs.python.org 的托管和构建流程，然后将其与实验性的 python.readthedocs.io 设置进行比较。特别关注发布历史、Python 2.0 及其他旧版本，以及所述的多版本、多语言和多格式构建负载。完成标准是制定出一份具体的迁移计划，在保留这些要求的同时解决 PSF 与托管之间的权衡。