Move all ASF CMS website content to GitHub

Recent changes in Apache infrastructure has made it impossible to use the existing ASF CMS system to manage the Apache Thrift web site. We need to extract and move the content somewhere else. For reference, see:

https://issues.apache.org/jira/browse/INFRA-17519

This is a bunch of work nobody was expecting to have to do.

My recommendation is to put the documentation into the "docs/" (or leave it in "doc/", perhaps, it if works) and use https://readthedocs.org/ to create the documentation web site. Documentation updates can be pushed into the repository with `[ci skip]` or maybe we teach GitHub and/or our Appveyor and Travis to do nothing on pull requests that only contain updates to markdown and/or rst files.