rethink how spelling warning is avoided

In reference to https://github.com/opendatakit/docs/pull/532#issuecomment-367795370, https://github.com/djungelorm/sphinx-tabs/pull/21

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Edit from @adammichaelwood ---

The spell-check builder has a conflict with the tabs feature.
My solution to fixing this was a bit hacky (copy spelling errors into file and then raise error if file is not empty). What I didn't know at the time was (as noted below) there is a better way to do this (suppressing warnings coming from the tabs extension).

This is low priority, but we should probably do it at some point because I think it is long-term easier to think about (and therefore easier to maintain).

Start with the linked docs pull request and the sphinx-tabs pull request, then trace the spell-check builder and tabs-extension warning interaction described in the issue. Done means replacing the current copied-error-file workaround with warning suppression and confirming the documentation spell-check build still reports spelling errors correctly.

Written by the indexing model from the issue text.