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ARROW-4511: [Format][Docs] Revamp Format documentation, consolidate columnar format docs into a more coherent single document. Add Versioning/Stability page

I made an effort to de-cruft the Format documentation, improve outdated, redundant, or unnecessary language, and avoid copy-pasting of stuff from the Flatbuffers files. I expect that readers of this document will be able to refer to the Flatbuffers files themselves (as long as it's clear where to look).

I also added a format/Versioning.rst which encodes the outcomes of our recent vote about how versions will be handled starting with 1.0.0. I am not sure where we'll write down the versions yet.

I maintained links from the old pages (which are hidden from the main toctree) so that stale search engine results will still lead people to the right place.

There's a bunch of JIRA issues connected to this. I assigned myself the ones that seemed relevant.

Closes #5202 from wesm/format-docs-revamp and squashes the following commits:

a1e8e870e <Wes McKinney> Review comments, move Integration Testing into a separate document
47a33967c <Wes McKinney> Revamp Format documentation, create single cohesive specification document. Add Versioning page

Authored-by: Wes McKinney <wesm+git@apache.org>
Signed-off-by: Wes McKinney <wesm+git@apache.org>