Move the PEPs to Read the Docs

[brettcannon]

Various things need to happen (as outlined as issues in https://github.com/python/peps/issues), but getting the PEPs to build instantly would be good. Could even create peps.python.org.

[brettcannon]

I created https://github.com/python/peps/projects/1 to track the work.

[hugovk]

Since this issue was opened, we now have PEPs built instantly, hosted on GitHub Pages at https://peps.python.org. We use RTD for build previews.

Shall we close this in favour of python/docs-community#5 which has more discussion?

[JelleZijlstra]

Yes, I think this can be tracked elsewhere.