ARROW-7912: [Format] C data interface

This version of the C data interface spec uses two distinct structures -- one for communicating a schema, one for communication array or record batch data.
This allows more flexibility in provider API schemes.

Closes #6040 from pitrou/doc-c-data-interface2 and squashes the following commits:

bd5dc28 <Neal Richardson> Remove duplicate import
7f81b8a <Antoine Pitrou> C Data Interface (take 2)

Lead-authored-by: Antoine Pitrou <antoine@python.org>
Co-authored-by: Neal Richardson <neal.p.richardson@gmail.com>
Signed-off-by: Neal Richardson <neal.p.richardson@gmail.com>

docs/source/conf.py

@@ -30,12 +30,14 @@
 # add these directories to sys.path here. If the directory is relative to the
 # documentation root, use os.path.abspath to make it absolute, like shown here.
 #
-import pyarrow
+
 import os
 import sys
-
 from unittest import mock
 
+import pyarrow
+
+
 sys.path.extend([
     os.path.join(os.path.dirname(__file__),
                  '..', '../..')