ARROW-16966: [Doc] Document Substrait conformance (#13494)
ARROW-16966: [Doc] Document Substrait conformance (#13494)

Authored-by: Weston Pace <0dd0706858a011bb30a3fd82b71cee4f9094c502@gmail.com>
Signed-off-by: Antoine Pitrou <9359a4d812173b65a3a0094cd86363e79731a3c2@python.org>
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