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ARROW-4339: [C++][Python] Developer documentation overhaul for 0.13 release

This was pretty much a huge pain but addresses accumulated documentation debt after the conda compiler migration and the CMake refactor. I suggest we not stress too much over small details on this and do more work to improve these docs in follow up PRs. I did the best I could under the circumstances and need to move on to other things now

I think the overall organization of the Sphinx project for developers is much improved, take a look (I will post a link to a published version for review)

JIRAs addressed by this PR and other things I did

* Update cpp/thirdparty/README.md given CMake refactor (this was totally out of date). This now directs users to the Sphinx C++ developer guide

* ARROW-4339: Move cpp/README.md to Sphinx documentation (and clean it up a lot!!)
* ARROW-4425: Move Contributing Guidelines from Confluence to Sphinx, update top level README
* ARROW-4232: Remove references to pre-gcc5 ABI issues
* ARROW-4165: Move Windows C++ developer guide to Sphinx (from cpp/apidoc/Windows.md)
* ARROW-4547: Update Python development instructions re: producing CUDA-enabled pyarrow
* ARROW-4326 / ARROW-3096: Update Python build instructions re: January 2019 compiler migration

Author: Wes McKinney <wesm+git@apache.org>

Closes #3942 from wesm/developer-docs-0.13 and squashes the following commits:

a3c3dd5de <Wes McKinney> Add some Boost info, misc cleaning
2ccc3de18 <Wes McKinney> Remove index.md altogether
66da97e7f <Wes McKinney> Remove unused text from cpp/apidoc/index.md
504bc134e <Wes McKinney> restore 'what's in the arrow libraries' section
8d1f33e19 <Wes McKinney> Finish initial documentation revamp for 0.13, stopping here
84dd680a2 <Wes McKinney> Some docs reorg, begin rewriting cpp/README.md into docs/source/developers/cpp.rst